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

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

  • 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

COVID Vaccine
A vaccine vial.

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

woman receiving covid vaccine
A woman receives a COVID-19 vaccine in Wales.

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.

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

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

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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Why we don’t have birth control for men

  • The condom was invented more than 5,000 years ago. While it’s made some strides since then, men are still left with few options for birth control, besides a vasectomy.
  • It’s not due to a lack of interest, but a lack of funding for research – and biology.
  • Men produce millions of sperm each day. Even if that count is reduced by 90%, they could still be fertile.
  • There are new methods coming, but experts say it will be years, if not decades, before they hit the market.
  • Visit BusinessInsider.com for more stories.

Following is a transcript of the video.

Narrator: Did you know the condom is over 5,000 years old? That’s right, some of the first forms of birth control date back to around 3,000 BC, and while the condom has made some strides since the Bronze Age, men still don’t have a much better option all these millennia later. Besides a vasectomy. Especially compared to the pills, IUDs, and implants available to women today. So why don’t we have birth control for men? In 2002, researchers asked more than 9,000 men across four continents whether they’d be willing to use contraception capable of preventing sperm production. Over half said yes. So the problem isn’t lack of interest, it’s partly human biology. Women ovulate just one or two eggs each month. Men, on the other hand, produce sperm daily, and it’s not just one or two.

There’s literally hundreds of millions of sperm produced each day, so because there’s so many sperm produced, actually, you can reduce your sperm number over 90% and still be completely fertile. Narrator: To reach infertility, a man’s sperm count needs to be somewhere around 1 to 10 million per milliliter, but getting there is near impossible, at least without side effects. That’s because sperm count is tied to the production of testosterone. In the past, researchers tried decreasing testosterone in an effort to decrease sperm count.

The problem is you don’t have any libido, you have very little testosterone to act on other tissues and so forth, and so the side effects were so dramatic that it really wasn’t ever going to be a contraceptive pill. Narrator: Scientists also tried using different compounds that attack the cells that produce sperm. But again, biology got in the way. Germ cells, as they’re called, developed inside a fortress-like structure within the testes.

So literally, nothing can get through it. There’s been a lot of small molecule studies to try and actually attack the germ cell to stop it from working. Literally, I can think of 10 or 15 different compounds that actually have been developed to do that, but they don’t work because of that barrier. Narrator: But the complex male anatomy isn’t the only problem. It’s also funding or lack thereof. In 2002, two big pharmaceutical companies took interest in male contraception, Schering and Organon. And together they funded a large clinical trial on a hormone-based contraceptive, offering hope that a pill backed by Big Pharma might be on the horizon.

Then these two companies became, as you know, acquired by bigger company, and then even bigger company, so now they are merged in huge companies, and women’s health is still a priority in many of the companies, but men’s health became part of the general matter of health. And therefore, the development of contraception becomes a really very low priority. Narrator: According to Dr. Wang, male contraception was also too risky for Big Pharma at the time. The long-term side effects were unknown. Companies were concerned that women might not trust it, and despite the survey results, it was unclear whether men would actually use a hormone-based contraception. Today, the limited funding comes mostly from government agencies like the National Institutes of Health. But there are in fact some promising lines of research. Dr. Wang is working on a gel that can lower testosterone where it matters, in the testes where sperm is produced, while keeping testosterone levels normal elsewhere. That means low sperm count and, more importantly, no major side effects.

We have preliminary studies to show that if we give the gel and if the man applies the gel, 90% of the men will reach the level that you talk about, 1 million per mil.

And Skinner is pursuing a new approach, shutting down Sertoli cells, which are a part of that impenetrable barrier that houses germ cells.

So if you shut down the Sertoli cell, then you shut down the sperm production. Narrator: But perhaps most promising is a sort of reversible vasectomy that’s in the works.

So they have this ability to inject this gel into what’s called the vas deferens, and it makes this plug, so then essentially it does the same thing, but you’re not cutting it. Then believe it or not, you can actually inject this chemical mixture, which will dissolve the plug, and so then you can get your fertility back. Narrator: But as promising as these approaches may be, they’re still years, if not decades, out, Skinner says. And without more funding, some of them may never hit the market. So at least for now, men are left with few options. Irreversible vasectomies, pulling out , and that slightly updated Bronze Age invention.

EDITOR’S NOTE: This video was originally published in April 2019.

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