Covid-19 Vaccine Side Effects: Your Questions Answered

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Every day nearly three million people in the United States are getting the Covid-19 vaccine. And every new jab prompts new questions about what to expect after vaccination.

Last week I asked readers to send me their questions about vaccinations. Here are some answers.

Short-lived side effects like fatigue, headache, muscle aches and fever are more common after the second dose of both the Pfizer-BioNTech and the Moderna vaccines, which each require two shots. (The Johnson & Johnson vaccine requires only a single shot.) Patients who experience unpleasant side effects after the second dose often describe feeling as if they have a bad flu and use phrases like “it flattened me” or “I was useless for two days.” During vaccine studies, patients were advised to schedule a few days off work after the second dose just in case they needed to spend a day or two in bed.

Data collected from v-safe, the app everyone is encouraged to use to track side effects after vaccination, also show an increase in reported side effects after the second dose. For instance, about 29 percent of people reported fatigue after the first Pfizer-BioNTech shot, but that jumped to 50 percent after the second dose. Muscle pain rose from 17 percent after the first shot to 42 percent after the second. While only about 7 percent of people got chills and fever after the first dose, that increased to about 26 percent after the second dose.

The New York Times interviewed several dozen of the newly vaccinated in the days afterward. They recounted a wide spectrum of responses, from no reaction at all to symptoms like uncontrolled shivering and “brain fog.” While these experiences aren’t pleasant, they are a sign that your own immune system is mounting a potent response to the vaccine.

An analysis of safety data from the first 13.7 million Covid-19 vaccine doses given to Americans found that side effects were more common in women. And while severe reactions to the Covid vaccine are rare, nearly all the cases of anaphylaxis, or life-threatening allergic reactions, occurred in women.

The finding that women are more likely to report and experience unpleasant side effects to the Covid vaccine is consistent with other vaccines as well. Women and girls can produce up to twice as many antibodies after receiving flu shots and vaccines for measles, mumps and rubella (M.M.R.) and hepatitis A and B. One study found that over nearly three decades, women accounted for 80 percent of all adult anaphylactic reactions to vaccines.

While it’s true that women may be more likely to report side effects than men, the higher rate of side effects in women also has a biological explanation. Estrogen can stimulate an immune response, whereas testosterone can blunt it. In addition, many immune-related genes are on the X chromosome, of which women have two copies and men have only one. These robust immune responses help to explain why 80 percent of autoimmune diseases afflict women. You can read more about women and vaccine side effects here.

Side effects get all the attention, but if you look at the data from vaccine clinical trials and the real world, you’ll see that many people don’t experience any side effects beyond a sore arm. In the Pfizer vaccine trials, about one out of four patients reported no side effects. In the Moderna trials, 57 percent of patients (64 or younger) reported side effects after the first dose — that jumped to 82 percent after the second dose, which means almost one in five patients reported no reaction after the second shot.

A lack of side effects does not mean the vaccine isn’t working, said Dr. Paul Offit, a professor at the University of Pennsylvania and a member of the Food and Drug Administration’s vaccine advisory panel. Dr. Offit noted that during the vaccine trials, a significant number of people didn’t report side effects, and yet the trials showed that about 95 percent of people were protected. “That proves you don’t have to have side effects in order to be protected,” he said.

Nobody really knows why some people have a lot of side effects and others have none. We do know that younger people mount stronger immune responses to vaccines than older people, whose immune systems get weaker with age. Women typically have stronger immune responses than men. But again, these differences don’t mean that you aren’t protected if you don’t feel much after getting the shot.

Scientists still aren’t sure how effective the vaccines are in people whose immune systems may be weakened from certain medical conditions, such as cancer treatments or H.I.V. infection or because they are taking immune suppressing drugs. But most experts believe the vaccines still offer these patients some protection against Covid-19.


April 1, 2021, 11:02 p.m. ET

The bottom line is that even though individual immune responses can vary, the data collected so far show that all three vaccines approved in the United States — Pfizer-BioNTech, Moderna and Johnson & Johnson — are effective against severe illness and death from Covid-19.

You shouldn’t try to stave off discomfort by taking a pain reliever before getting the shot. The concern is that premedicating with a pain reliever like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin), which can prevent side effects like arm soreness as well as fever or headache, might also blunt your body’s immune response.

While it’s possible that taking a pain reliever before your shots might have dampened your body’s immune response, vaccine experts say you shouldn’t worry, and you shouldn’t try to get another shot. Studies of other vaccines suggest that while premedicating can dull the body’s immune response to a vaccine, your immune system can still mount a strong enough defense to fight infection. A review of studies of more than 5,000 children compared antibody levels in children who took pain relievers before and after vaccinations and those who did not. They found that pain relievers did not have a meaningful impact on immune response, and that children in both groups generated adequate levels of antibodies after their shots.

The high efficacy of all the Covid vaccines suggests that even if taking Tylenol before the shot did blunt your body’s immune response, there’s some wiggle room, and you are likely still well protected against Covid-19. “You should feel reassured that you’ll have enough of an immune response that you’ll will be protected, especially for vaccines that are this good,” said Dr. Offit.

“It’s OK to treat” side effects with a pain reliever, said Dr. Offit, but if you don’t really need one, “don’t take it.”

While most experts agree it’s safe to take a pain reliever to relieve discomfort after you get vaccinated, they advise against taking it after the shot as a preventive or if your symptoms are manageable without it. The concern with taking an unnecessary pain reliever is that it could blunt some of the effects of the vaccine. (In terms of the vaccine, there’s no meaningful difference if you choose acetaminophen or ibuprofen.)

During the Moderna trial, about 26 percent of people took acetaminophen to relieve side effects, and the overall efficacy of the vaccine still was 94 percent.

Research and anecdotal reports suggest that people with a previously diagnosed Covid-19 infection may have a stronger reaction and more side effects after their first dose of vaccine compared to those who were never infected with the virus. A strong reaction to your first dose of vaccine also might be a sign that you were previously infected, even if you weren’t aware of it.

If you previously tested positive for Covid-19 or had a positive antibody blood test, be prepared for a stronger reaction to your first dose, and consider scheduling a few days off work just in case. Not only will it be more comfortable to stay home and recover in bed, the vaccine side effects can resemble the symptoms Covid-19, and your co-workers won’t want to be near you anyway.

Studies suggest that one dose might be adequate for people who have a previously confirmed case of Covid-19, but so far the medical guidelines haven’t changed. If you’ve received the Pfizer-BioNTech or Moderna vaccines, you should plan to get your second dose even if you’ve had Covid-19. Skipping your second dose could create problems if your employer or an airline ask to see proof of vaccination in the future. If you live in an area where the single-dose Johnson & Johnson vaccine is available, then you can be fully vaccinated after just one dose. You can read more here about the vaccine response in people who’ve had Covid-19.

The vaccines appear to be effective against a new variant that originated in Britain and is quickly becoming dominant in the United States. But some variants of the coronavirus, particularly one first identified in South Africa and one in Brazil, appear to be more adept at dodging antibodies in vaccinated people.

While that sounds worrisome, there’s reason to be hopeful. Vaccinated people exposed to a more resistant variant still appear to be protected against serious illness. And scientists have a clear enough understanding of the variants that they already are working on developing booster shots that will target the variants. The variants identified in South Africa and Brazil are not yet widespread in the United States.

People who are vaccinated should still wear masks in public and comply with public health guidelines, but you shouldn’t live in fear of variants, said Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston. “If you’re vaccinated, you should feel pretty confident about how protected you are,” said Dr. Hotez. “It’s unlikely you’ll ever go to a hospital or an I.C.U. with Covid-19. In time you’re going to see a recommendation for a booster.”

I hope these answers will reassure you about your own vaccine experience. You can find a more complete list of questions and answers in our special vaccine tool “Answers to All Your Questions About Getting Vaccinated Against Covid-19.”