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Johnson & Johnson had a very bad week – but fears of negative reactions and blood clots are likely overblown

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Nurse Elizabeth Johnson administers a COVID-19 vaccine to Melissa Mendez in Reading, Pennsylvania.

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Johnson & Johnson’s coronavirus vaccine rollout hit several unfortunate snags this week – some far worse than others.

The New York Times reported on Tuesday that 62 million of the company’s vaccine doses must be checked for contamination, following an error at a Maryland manufacturing plant that already ruined 15 million doses. The plant’s workers accidentally mixed up some vaccine ingredients last month.

Then a vaccination site in Colorado, three sites in North Carolina, and one in Georgia all temporarily stopped administering Johnson & Johnson’s shot after about 45 people in total experienced minor adverse reactions involving nausea, dizziness, fainting, or lightheadedness.

To top it off, European regulators announced Friday that they are investigating Johnson & Johnson’s shot for links to unusual blood clotting after four blood-clot cases, including one death, were reported among vaccine recipients.

The timing of these developments was unfortunate, experts said, but there’s no reason to doubt the shot’s safety yet.

“You don’t want to be fueling unnecessary worries about the safety of vaccines when you’re still seeing an enormous outbreak and death rates all over the world from COVID,” Art Caplan, a bioethicist at New York University, told Insider.

He added that the side effects observed at the US vaccination sites were “absolutely trivial.”

Although symptoms like nausea and fainting aren’t common responses to coronavirus vaccines, they aren’t abnormal, either.

“When you see these clusters [of side effects], they usually are worked out and have no relation to the vaccine,” Arnold Monto, an epidemiologist at the University of Michigan’s School of Public Health, told Insider. Monto chairs the Food and Drug Administration’s Vaccine Advisory Committee, which voted to recommend the Pfizer, Moderna, and Johnson & Johnson shots.

People could get dizzy or nauseous for many reasons

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Kristine Ko, right, waits in line for her vaccine at the Cal State LA walk-up mass vaccine site in Los Angeles on April 8.

Al Seib/Los Angeles Times/Getty Images

Most of the negative reactions to Johnson & Johnson’s shot occurred within 15 minutes after a person got vaccinated, public-health officials in Colorado, Georgia, and North Carolina reported. In total between the three states, seven people were taken to hospitals for observation. As of Friday, all but one had been released and everyone was expected to fully recover.

Officials haven’t yet pinpointed why these clusters of adverse reactions were identified in such a short time frame. But they’ve emphasized that there’s no reason to be concerned about the shot itself.

In a news release, Georgia’s public health commissioner, Kathleen Toomey, said the state was looking into “what may have caused the reactions, including the conditions at the fairgrounds such as heat and the ability to keep the site cool.”

Caplan said it’s also possible that the people who felt dizzy or nauseous were elderly or had underlying health conditions that may have predisposed them to adverse reactions. And once one site reports a cluster of patient reactions, that makes others more likely to look for them and take swift action.

Whatever the reason, Monto said, the fact that adverse reactions are being reported and investigated shows that local officials are working hard to minimize harm.

“Any pause in vaccination shows that the system is working because we’re not trying to sweep anything under the rug,” he said.

In a statement, Johnson & Johnson told Insider that it takes all adverse reactions seriously. The company said it would “carefully assess the events” and share any findings with the FDA.

“There is no greater priority than the safety and well-being of the people we serve,” the statement said.

The link between J&J’s shot and blood clots remains tenuous

johnson & johnson vaccine
A vial of Johnson & Johnson’s shot.

Michael Ciaglo/Getty Images

Just four blood-clot cases have been identified among people who’ve gotten Johnson & Johnson’s shot so far. In one of those cases, a participant in the company’s clinical trial died of a clotting disorder.

The other three blood-clot cases occurred during J&J’s US rollout, which has administered nearly 5.4 million doses of its vaccine. That’s fewer than one clot case per 1 million shots.

Experts aren’t convinced there’s a cause-and-effect relationship at all, though.

“It isn’t really clear how many people get blood clots anyway,” Caplan said. ‘That makes it hard to know whether it’s connected to vaccination.”

The European Union’s health regulator, the European Medicines Agency (EMA), announced its investigation after receiving information about the blood-clot cases from the FDA, Bloomberg reported. The EU authorized Johnson & Johnson’s vaccine in March and is scheduled to roll out the shot later this month.

The agency said it wasn’t sure what relationship, if any, existed between the vaccine and blood clots.

Earlier this week, the EMA announced that potentially fatal blood clots might be a rare side effect of AstraZeneca’s vaccine, which has been authorized in more than 110 countries but not yet in the US. Both that shot and Johnson & Johnson’s are viral-vector vaccines, which introduce a coronavirus gene into the body using a genetically engineered common-cold virus. Scientists don’t know whether the platform itself could be linked to clotting.

Still, any risk of clots should be weighed against the risk of getting COVID-19, Caplan said.

“If I live in Brazil and I’m still seeing 3,000 deaths a day, and somebody said to me, ‘We’re going to pull AstraZeneca vaccine out of here because we found somebody who died of a blood clot that might be related to the vaccine,’ I’d say, ‘You better recalculate your risk-benefit ratio,'” he said.

The bigger problem is a lack of supply

For now, the most pressing issue with Johnson & Johnson’s vaccine isn’t any safety concern – it’s the lack of consistent supply.

Johnson & Johnson promised to ship 24 million doses in US this month, but it’s unclear if the company will meet that goal, given the recent manufacturing issues, which occurred at a plant run by Emergent Biosolutions, a US government contractor.

Quality-control managers must first test 62 million doses to see if they can be salvaged, and the FDA must also certify that the Maryland plant can release more doses to the public.

Jeff Zients, the White House COVID-19 response coordinator, said at a Friday briefing that Johnson & Johnson “expects a relatively low level of weekly dose delivery” until the FDA signs off on the plant.

The total number of Johnson & Johnson shots allocated to states and other jurisdictions is expected to drop around 85% next week, to around 1.5 million doses. But that decrease follows an unprecedented surge of roughly 11 million doses last week.

Johnson & Johnson has said it’s still on track to provide close to 100 million doses by the end of May.

Allison DeAngelis contributed reporting.

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The COVID-19 vaccine side effects you can expect based on your age, sex, and dose

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Alameda County workers line up to receive coronavirus vaccines outside St. Rose Hospital in Hayward, California, on January 8, 2021.

Jessica Christian/The San Francisco Chronicle/Getty Images

  • COVID-19 vaccine side effects can vary depending on a person’s age, sex, or health status.
  • Women and younger adults tend to have more side effects than men or older adults do.
  • Side effects are generally more pronounced after the second dose than the first.
  • See more stories on Insider’s business page.

When Freedom Baird got her first dose of Moderna’s vaccine in February, she wasn’t sure what kind of side effects to anticipate.

Baird is a COVID-19 long-hauler – she’s had lingering shortness of breath and chest pain for roughly a year. Many people who’ve had a prior infection develop more side effects in response to the first vaccine dose than the second. An average person, meanwhile, typically feels more run-down after their second shot.

Baird’s age complicated her expectations: She’s 56, and clinical trials have shown that people over 55 often develop fewer vaccine side effects. As it turns out, she didn’t feel much.

“It was really just that first day of feeling achy and flu-y,” Baird told Insider.

While doctors can’t predict exactly how someone will respond to a coronavirus vaccine, they’ve identified a few patterns based on a person’s age, sex, health status, and which dose they’re receiving. Clinical trials suggest that side effects are generally more pronounced among women and younger adults, especially after their second dose.

Dose two usually comes with more severe side effects

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A vaccine vial.

Jessica Rinaldi/The Boston Globe via Getty Images

The most common side effect for all three authorized US vaccines is pain or swelling at the injection site: Nearly 92% of participants in Moderna’s clinical trial developed this side effect. in Pfizer’s trial, 84% of participants reported that, as did 49% in Johnson & Johnson’s.

Other common side effects include fatigue, headache, and body or muscle aches. Around 65% of vaccine recipients in Pfizer’s and Moderna’s trials, and 38% in Johnson & Johnson’s, developed fatigue.

For those who haven’t had COVID-19 before, side effects tend to be more numerous and severe after the second dose.

Roughly twice as many participants in Pfizer’s trial developed chills and joint pain after their second dose than after their first. In Moderna’s trial, meanwhile, around five times as many participants developed chills after their second dose than their first. Fevers were also far more common among second-dose recipients than first-dose recipients in both trials.

People who’ve had COVID-19 may develop more side effects after dose 1

A small study from the Icahn School of Medicine at Mount Sinai found that vaccine side effects such as fatigue, headaches, and chills were more common among people with pre-existing immunity to the coronavirus than people who’d never been infected before. Around 73% of vaccine recipients who’d previously had COVID-19 developed side effects after dose one of Pfizer’s or Moderna’s shot, compared to 66% of vaccine recipients who’d never gotten infected before.

“If you’ve already had a COVID-19 infection, you’ve developed memory cells from that infection,” Dr. Vivek Cherian, a Baltimore-based internal medicine physician, told Insider.

“If you were ever to be exposed to the infection again, your body would basically be able to respond quickly and more robustly that second time around,” he added. “That’s why you tend to have more strong side effects from that initial vaccine.”

Younger adults may feel more run-down after their shots than older people

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A woman receives a COVID-19 vaccine in Wales.

Getty/Matthew Horwood

Our immune systems gradually deteriorate as we age, which means older people’s bodies don’t work as hard to defend them against foreign invaders – including the protein introduced to the body via a vaccine.

“Younger individuals have a much more vigorous immune response, so it should make sense that they would also have more side effects,” Cherian said.

After one dose of Moderna’s shot, 57% of people younger than 65 developed side effects, compared to 48% of those older than 65. After the second dose, nearly 82% of people in the younger group developed side effects, compared to nearly 72% of older adults.

Pfizer broke down its data slightly differently: Around 47% of people ages 18 to 55 developed fatigue after dose one, whereas 34% of people ages 56 and older reported that side effect. After dose two, the numbers rose to 59% and 51%, respectively.

After Johnson & Johnson’s one-shot vaccine, nearly 62% of people ages 18 to 59 developed side effects, compared to 45% of people ages 60 and up.

Women can expect more side effects in general

coronavirus vaccine recipient
A woman receives a COVID-19 vaccine.

John Moore/Getty Images

The Centers for Disease Control and Prevention analyzed Americans’ reactions to nearly 14 million doses of the Pfizer and Moderna shots from December to January. The results showed that roughly 79% of instances of vaccine side effects reported to the CDC came from women, though just 61% of doses were administered to women overall.

Cherian said women tend to react more strongly to vaccines for polio, influenza, measles, and mumps as well.

“All of these vaccines in general, women tend to have greater side effects,” he said. “They’re even more pronounced for a pre-menopausal woman compared to a post-menopausal woman.”

Scientists suspect the difference has to do with estrogen levels.

“Testosterone tends to be an immune-suppressive hormone and estrogen tends to be an immune stimulant,” Cherian said. “So more than likely it’s the estrogen hormone – that’s why females tend to have more side effects.”

Most high-risk medical conditions won’t lead to stronger side effects

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CVS pharmacist Gina Glancy gives the second dose of Pfizer’s COVID-19 vaccine to Bob Jensen, 89, in West Chester, Pennsylvania.

Pete Bannan via Getty Images

People with weakened immune systems don’t mount a strong defense against viral infections in general, so they’re particularly vulnerable to severe COVID-19. For that reason, the CDC recommends that these groups get vaccinated right away.

But it’s possible that immunocompromised people, such as cancer patients, won’t mount a strong immune response to the vaccine, either.

“Your immune response essentially dictates your side effects, so if you’re immunocompromised, you may not necessarily be having as many side effects, but you should still absolutely get vaccinated,” Cherian said.

The vaccines should provide immunocompromised people with at least some protection against severe COVID-19, even if they don’t feel any side effects – though the effectiveness may be lower than for the average person.

Cherian said that for people with autoimmune conditions, meanwhile, the side effects probably won’t be any worse than for the average person.

“If you have those high risk factors, you really, really want to get vaccinated,” he said. “Dealing with a few side effects of some diarrhea or some muscle aches is a much, much better thing than some of those serious, potentially life-threatening side effects of the COVID-19 infection.”

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5 people share their Johnson & Johnson vaccine side effects, from sore arms to chills

Johnson & Johnson coronavirus vaccine clinical trial, volunteer dosed with experimental COVID-19 vaccine
A clinical trial volunteer participates in Johnson & Johnson’s study to test a coronavirus vaccine.

Janssen

  • All of the COVID-19 vaccines come with the possibility of side effects.
  • Fewer Johnson & Johnson recipients had reactions compared to people in other vaccine trials.
  • Still, Insider spoke with 5 people who had arm pain, chills, or headaches after the J&J shot.
  • See more stories on Insider’s business page.

Chloe Kathuria, an 18-year-old college student, wrapped herself in a dozen blankets the night she got the Johnson & Johnson vaccine.

“I was freezing,” Kathuria told Insider. “For hours I couldn’t stop moving because I was shaking and that was uncomfortable because all my muscles were hurting.”

Kathuria is one of five J&J vaccine recipients who told Insider they experienced side effects after getting the jab. Others have reported unpleasant reactions to the Moderna and Pfizer shots, so no vaccine is without its drawbacks.

Although no one wants to get a fever and chills, experts say these side effects are nothing to be concerned about. Most reactions will go away within a couple of days, and they’re proof that your immune system is doing its job.

If you want to plan ahead, you should prepare to take it easy for a day or two after you get your shot. Kathuria had to skip class the day after her shot, and other recipients said they called in sick from work and other obligations, before returning later in the week.

It’s normal to experience some side effects after a vaccine

Side effects including arm pain, headaches, and fatigue are common post-vaccine. All are signs that your immune system is learning to recognize and respond to the coronavirus, Akiko Iwasaki, who researches viruses at Yale, previously told Insider’s Hilary Brueck.

“When these things are being taken up into a cell, they really trigger sensors that are there to detect viruses,” Iwasaki said. “So you might feel sore in the arm, a little fever. These are usual responses to this sort of innate immune activation.”

Kathuria felt all of the most commonly reported vaccine side effects, and then some: arm pain, dizziness, bad chills, headache, loss of appetite, muscle aches, and fever. Her fever lasted two days, and her arm pain and muscle aches lasted a week.

woman gets johnson & johnson vaccine
A woman receives a Johnson & Johnson Covid-19 vaccine from registered nurse Gina Reed at a vaccination center established at the Hilton Chicago O’Hare Airport Hotel in Chicago, Illinois, on March 5, 2021.

Kamil Krzaczynski/AFP via Getty Images

But not everyone experiences a slew of side effects. In trials of the Johnson & Johnson vaccine, less than 50% of people reported arm pain after their injections, which is notably lower than 91% of Moderna vaccine recipients with achy arms. An even smaller portion – roughly 38% – of J&J recipients reported headaches and fatigue.

Laura Mclean, 57, didn’t feel any pain in her arm post-injection. However, her side effects started in the middle of the night, when she woke up with chills and a headache. She said she still had chills in the morning and left a volunteer shift early, but fully recovered by dinnertime.

Some people waited for the J&J vaccine in hopes of avoiding side effects

Linda Davis, 73, was eligible to take the Pfizer and Moderna vaccines early on, but she waited until the J&J approval to get vaccinated.

Davis has an autoimmune disease called fibromyalgia, and the medications she’s taken to treat it haven’t sat well.

“It was like somebody took a baseball bat, wound up as hard as they couldn’t hit me in the stomach,” she said of previous reactions.

She feared the Pfizer and Moderna vaccines would cause a severe reaction after reading that other people experienced nasty side effects. In comparison, J&J’s trial participants reported fewer serious reactions.

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Participants in the Johnson & Johnson vaccine trial reported fewer side effects compared to those who got the Moderna and Pfizer shots.

Artur Widak/NurPhoto via Getty Images

Davis said she “moved heaven and earth” to find a provider who had the J&J vaccine, which meant spending hours on the phone and online. When she finally found the shot, she experienced some arm pain for two to three days after the injection, but no serious side effects.

Pat Wyman also waited for the J&J vaccine because she’s allergic to PEG, an ingredient in both the Pfizer and Moderna vaccine. Following her doctor’s recommendations, she found a pharmacy with the J&J vaccine and got vaccinated.

After vaccination, Wyman had a high fever, chills, and a headache that lasted up to five days.

“I would do it again in a heartbeat because it is so important for me to feel safer in general,” Wyman previously told Insider.

The side effects were worth getting vaccinated

All of the J&J recipients who spoke with Insider said the temporary side effects were worth the promised return to normalcy.

Christian Mendonca felt tired, cold, and ran a fever 12 hours after his J&J vaccine. Three days after his shot, Mendonca still can’t comfortably raise his vaccinated arm above his shoulder – but he said getting the vaccine was worth it.

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Cristi Hunkler, a retired school teacher receives the vaccine on the first day of the Johnson and Johnson vaccine being made available to residents of Athens, Ohio.

Stephen Zenner/SOPA Images via Getty Images

“My wife and I are actually also trying to get pregnant, so just having that one weight off our shoulders before we try to welcome a child into the world is a huge benefit,” Mendonca told Insider.

Once Davis is fully immune to COVID-19, she plans to “scoop my grandchildren up and hold them close.”

Even Kathuria, whose muscle aches and arm pain lasted a week, said the side effects were worth getting vaccinated.

“Two days of a fever is better than having the coronavirus and being hospitalized.”

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Two 13-year-old brothers are participating in Moderna’s vaccine trial for kids. They’re ‘pretty chill’ about it.

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Gavin Preston (left) and Emmett Preston (right).

Katie Preston

  • Emmett and Gavin Preston, 13-year-old brothers, are participants in Moderna’s vaccine trial.
  • Moderna is studying the effects of its coronavirus shot among 3,000 kids from 12 to 17.
  • The brothers said they didn’t want to spread the virus to their mom, who has an autoimmune condition.
  • See more stories on Insider’s business page.

At 13 years old, Emmett and Gavin Preston aren’t yet eligible to receive a coronavirus vaccine. The Food and Drug Administration hasn’t approved the shots for kids under 16 due to a lack of data.

But the Preston brothers are participants in Moderna’s clinical trial: The company is currently studying the effects of the shot among 3,000 kids ages 12 to 17. A separate trial is testing it in 6,750 children under 12, including babies as young as six months.

Emmett and Gavin, both adopted, received their first shots on February 12 at their local doctor’s office in Charleston, South Carolina, then their second shots one month later. The brothers said they were a bit nervous about the needle but haven’t felt many side effects.

“When they showed me the shot, I was like, ‘What?’ And then when I took it, I was like, ‘Oh, that’s not that bad,'” Emmett told Insider.

For a week after each dose, they logged their symptoms into an app so researchers could track their side effects.

“I just felt like my normal self,” Emmett said. By the second time he got vaccinated, he added, he was “pretty chill” about it.

Kids may respond differently to the vaccine than adults

Moderna vaccine
A nurse prepares a coronavirus vaccine shot developed by the National Institutes of Health and Moderna Inc., July 2020.

Hans Pennink/AP

Moderna’s trials are randomized, double-blind studies, meaning half of the participants get the vaccine while the other half get a placebo, and nobody knows which they’ve gotten.

For some adult participants, side effects were sometimes a clue that they’d gotten more than a saline solution – some felt headache, fever, or chills (indicators that their bodies were reacting and building antibodies). But scientists aren’t sure whether kids will feel side effects to the same degree.

“Children generally respond well to vaccines,” Donna Farber, a Columbia University immunologist, previously told Insider. “They should respond well or comparably to a young adult – and maybe even better.”

Emmett said he had a sore shoulder and “felt a bit run down” after the first shot. Gavin said he had some minor fatigue two days after the second one.

“It went perfectly after all that,” Gavin said. “Nothing has really happened.”

In general, kids aren’t very susceptible to severe illness due to COVID-19. Children represent around 13% of confirmed coronavirus infections in the US, but less than 0.2% of the nation’s coronavirus deaths. Some researchers suspect that’s because kids’ immune systems fight off the virus before it has a chance to replicate widely.

The Preston brothers were on board with the trial right away

Children can be difficult to include in trials, since ethical questions arise if they don’t fully understand what they’re signing up for, and scientists are cautious of any research that could stunt a child’s development. Many parents are not comfortable signing their kids up to participate for the same reasons.

But the Preston brothers are no strangers to clinical trials. For the past few years, they’ve participated in the Adolescent Brain Cognitive Development Study, the largest long-term study of brain development and child health in the US. The study recruits healthy children ages 9 to 10 and observes their brain growth through early adulthood.

So when the Preston boys’ mother, Katie, approached them about getting the Moderna vaccine, they didn’t think twice about it, they said.

“My mom brought it up and I was like, ‘Ooh, that sounds interesting,'” Emmett said.

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The Preston brothers wait to receive their shots.

Katie Preston

Katie said she learned of the trial through a friend who posted about it on Facebook. She emailed her doctor’s office in Charleston to express interest, then heard back a few weeks later.

“We had to go through a little screening process and they said the boys were good, so we went for it,” Katie said.

Emmett and Gavin are less stressed about in-person school now

school closure empty classroom
An empty classroom.

Tom Williams/CQ-Roll Call, Inc via Getty Images

The Preston brothers were motivated to get vaccinated as soon as possible because Katie has an autoimmune condition and both boys have returned to school in-person. They didn’t want bring the virus home.

Coronavirus transmission tends to be low in schools, as long as masks and social distancing are mandatory, but kids can still spread the virus, even if they have no symptoms.

“Just being able to protect myself and my family from coronavirus really makes me feel positive inside,” Gavin said.

He added that he was eager to assist with scientific research, since science is one of his favorite subjects.

When he grows up, he said, “I want to research dementia, Alzheimer’s, anything memory loss related, to try and help those people.”

Knowing that there’s a 50-50 chance they’re vaccinated, both brothers said, has made them less stressed about getting sick at school. Neither wants to return to virtual learning.

“It was really hard to learn online,” Gavin said. “Then when I got in school, my grades started to go up.”

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The US hasn’t authorized AstraZeneca’s vaccine for 2 main reasons. That could change in April.

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French Health Minister Olivier Veran receives the AstraZeneca-Oxford COVID-19 vaccine on February 8, 2021.

THOMAS SAMSON/AFP via Getty Images

  • The US hasn’t authorized AstraZeneca’s vaccine because its trial is still going.
  • Vaccine experts have also questioned inconsistencies in the company’s global studies.
  • AstraZeneca’s US trial results will likely clear up confusion in April.
  • See more stories on Insider’s business page.

After more than a dozen countries paused the use of AstraZeneca’s coronavirus vaccine due to concerns about blood clots, the European Medicines Agency concluded on Thursday that the vaccine doesn’t increase the risk of clotting.

The World Health Organization, too, recommended that countries continue to administer AstraZeneca’s vaccine, since the benefits of the shot “outweigh its risks.”

The US, however, never greenlit the vaccine in the first place, for two main reasons. The first is that AstraZeneca’s US trial is still ongoing. The trial was delayed for nearly seven weeks in the fall due to an adverse reaction in a UK participant.

The second is that AstraZeneca’s initial trial results in November puzzled many experts. The company combined data from its Brazil and UK groups, even though they had different sample sizes and demographics. It also averaged the results of two different dosing regimens, one of which was administered by mistake.

Now, scientists hope that results from AstraZeneca’s US trial will clear up much of the world’s confusion. The findings are expected to be released to the public within the next month.

Currently, the US data is being reviewed by an independent monitoring board. If the vaccine is found to be safe and effective, the FDA could authorize it for emergency use in April, Reuters recently reported.

The US trial has more than 30,000 participants

Even if AstraZeneca’s global trials had been free of inconsistencies, the FDA would most likely still have wanted to see the results of the US trial before authorizing the shot.

The US trial is larger than its predecessors in other countries. The study involves up to 32,000 volunteers – roughly the same number as the clinical trials done by Pfizer and Moderna. And it’s only testing one regimen: two full doses.

“The US study is the one that’s going to be the definitive study to tell us how this vaccine works against severe, hospitalized COVID and symptomatic COVID,” Anna Durbin, a vaccine researcher at Johns Hopkins Bloomberg School of Public Health, told STAT.

AstraZeneca CEO Pascal Soriot
Pascal Soriot, executive director and CEO of AstraZeneca, testifies before the Senate Finance Committee on February 26, 2019.

Photo by Win McNamee/Getty Images

The trial could also shed light on how the vaccine performs among older people. Nearly one-quarter of the US trial participants were over 65, according to Biopharma Dive. Meanwhile, just 12% of participants in AstraZeneca’s UK and Brazil trials were over 55.

Murky data from the UK and Brazil

The FDA requires coronavirus shots to prevent disease in at least 50% of vaccine recipients.

In November, AstraZeneca reported that its shot was 62% effective at preventing COVID-19 among nearly 9,000 volunteers in Brazil and the UK. Those participants received two full doses, but about 2,700 others accidentally got a half dose followed by a full dose. Among that smaller group, the vaccine’s effectiveness rose to 90%.

AstraZeneca averaged the two results, stating that the vaccine was 70% effective.

“To people looking in from the outside, that doesn’t make a whole lot of sense,” Dr. Johan Bester, director of bioethics at the University of Nevada, Las Vegas School of Medicine, told Insider last month. “We’re either going to give half doses or full doses to people and it will either be one effectiveness or the other.”

AstraZeneca’s shot may be less effective against the variant found in South Africa

In the months since AstraZeneca revealed its global trial results, subsequent analyses have found even more variation in the shot’s effectiveness.

A February analysis that hasn’t been peer reviewed found that the vaccine was 55% effective when doses were given less than six weeks apart, but 82% effective when doses were given at least 12 weeks apart. The results were based on trials in Brazil, South Africa, and the UK with more than 17,000 volunteers in total.

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AstraZeneca COVID-19 vaccines in storage in Copenhagen, Denmark on February 11, 2021.

Ritzau Scanpix/Liselotte Sabroe via REUTERS

Another study that’s still awaiting peer review indicated that the vaccine was just 22% effective against mild and moderate illness caused by B.1.351, the variant first identified in South Africa. As a result, South Africa halted its distribution of AstraZeneca’s vaccine. But that study was small, with roughly 2,000 participants.

When it comes to two other prominent variants – B.1.1.7, initially spotted in the UK, and P.1, first identified at a Japanese airport among travelers from Brazil – preliminary data shows AstraZeneca’s vaccine has the same efficacy: between 60% and 90%.

The US trial may not give any more clarity about how well the vaccine works against these three variants, since it’s not yet known how many volunteers wound up getting exposed to those strains during the study. But the fact the trial is happening later than Pfizer’s or Moderna’s means there’s a greater chance that participants in this one were exposed to new variants.

Even in the worst-case scenario, Bester said, the shot will probably be effective enough to meet FDA standards.

“What we’ve seen is promising,” he said. “It is a moderately to very effective vaccine that is cheaper than the other alternatives and gives us another tool in our arsenal to stop people from dying.”

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