Covid-19 Vaccine Works, Even if Side Effects Differ for All, Doctors Say – The Wall Street Journal

Infectious-disease specialists are working to reassure people that they are still getting protection from Covid-19 vaccines, even if they don’t experience the flulike side effects that hit some people after vaccination.

Fatigue, chills and other symptoms in the days following vaccination are evidence that the vaccine is having the desired effect on the body’s immune system, according to public health officials. The U.S. Centers for Disease Control and Prevention and the World Health Organization say on their websites that side effects mean the body is “building protection” against the coronavirus.

That message may lead some people to infer that the absence of side effects indicates that vaccination isn’t causing the body to build immunity to the virus. Yet infectious-disease doctors say most people get protection from the vaccines, even if they don’t experience side effects.

“I don’t think someone should correlate the extent of their reactions to the vaccine with protection from infection,” said H. Cody Meissner, chief of the pediatric infectious diseases division at Tufts University School of Medicine in Boston. “We know that people who don’t respond to a vaccine in terms of the side effects still are well protected. The vaccines work even if you don’t have fatigue and headache and fever and muscle pain and joint pain.”

Experts say more research is needed to establish what vaccine-related side effects, or their absence, tell us about the strength of people’s immune responses. “There are just so many nuances in terms of how you respond,” said Kathryn Edwards, professor of pediatrics and a vaccine researcher at Vanderbilt University School of Medicine in Nashville, Tenn.

Dr. Edwards said there is a biological basis to tell people “it’s good to feel bad” because side effects can be a sign of an immune response. But, she added, “I think we should have confirmation there is a relationship.” She said she has fielded questions from vaccine recipients who didn’t experience side effects and worried that the absence meant they weren’t getting protection.

Vaccines against other diseases have been known to cause side effects because the immune response releases inflammatory substances in the body.

A small study conducted recently by scientists at the University of Pennsylvania found that people who had more robust side effects after receiving either of the two leading vaccines in the U.S.—from Pfizer Inc. PFE -0.20% / BioNTech SE and Moderna Inc. —had slightly higher antibody levels than those who had less robust side effects. Yet all people getting the vaccine in the study had good immune responses, said study co-author E. John Wherry, director of the Penn Institute for Immunology.

The Pfizer and Moderna vaccines use RNA-based technology and can cause similar side effects. Ranging from injection-site pain, fatigue, headache, muscle pain, joint pain, in addition to fever and chills, these typically arise within a day of vaccination and resolve within a couple of days. The side effects can often be managed by taking acetaminophen or another pain reliever.

The vaccines were at least 94% effective at protecting against Covid-19 in separate, large clinical trials that started last year.

Experts say more research is needed to establish what vaccine-related side effects, or their absence, tell us about the strength of people’s immune responses.

Photo: jose luis gonzalez/Reuters

Injection-site pain or swelling is the most common reaction, occurring in 92% of Moderna vaccine recipients and 84% of those getting the Pfizer shot, the studies found.

A Pfizer spokeswoman said side effects don’t indicate the level of immunity conferred by its vaccine. It wouldn’t be able to demonstrate such high efficacy, if the only people protected were the ones with symptoms, she said.

Moderna didn’t immediately reply to a request for comment.

Johnson & Johnson’s JNJ 0.15% one-shot Covid-19 vaccine uses a different technology than the Pfizer and Moderna shots. A large study found that injection-site pain affected about 49% of recipients, while headache, fatigue or muscle ache occurred at lower rates. A small number of J&J vaccine recipients have developed serious blood clots.

Age appears to be a factor in determining who experiences side effects. People over age 65 are less likely than younger people to experience side effects. Older adults tend to have less robust immune responses to vaccines.

As more U.S. adults get their Covid-19 vaccines, a variety of side effects are emerging. WSJ’s Daniela Hernandez speaks with an infectious disease specialist on what is common, what isn’t and when to seek medical attention. Photo: Associated Press

Vanderbilt’s Dr. Edwards said women appear to be more likely than men to experience the side effects, which may be related to hormonal or weight differences.

The Food and Drug Administration says age, sex and general health likely influence the occurrence and severity of common side effects in the first couple of days after vaccination. Side effects don’t correlate with the effectiveness of a vaccine in an individual, an FDA spokeswoman said.

Some doctors say heredity may also play a role. “I am sure that our genes at some level determine differences in these responses,” Paul Offit, director of the vaccine education center at Children’s Hospital of Philadelphia, said of varying levels of symptoms following vaccination.

For some receiving the RNA vaccines, side effects are more pronounced after the second dose, making it hard for people to work or be active. In contrast, people who were previously infected with the coronavirus have had more pronounced side effects after the first dose, Dr. Edwards said.

Major Hayden, a 38-year-old software developer living near San Antonio, said he began feeling chills, fatigue and fever several hours after getting the second dose of Moderna’s vaccine in early May. He took acetaminophen and began to feel better the following day.

“For me it just seemed like the risks from the vaccine were much smaller than the risks of Covid,” he said.

Covid-19 Vaccines

Write to Peter Loftus at


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