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October 10, 2023

Feeling awful after getting a COVID-19 booster? That's a good sign, study says

Chills, fever, soreness and other side effects point to a more vigorous immune response, research shows. But even people who don't experience symptoms gain ample protection

People who experience fatigue, fever, muscle pain and chills after they get vaccinated for COVID-19 appear to have stronger and longer-lasting protection from serious coronavirus infections, according to a new study.

People who had multiple side effects were more likely to have higher antibody levels six months later than those who didn't experience symptoms from their shots, researchers found. Higher skin temperatures and raised heart rate particularly were tied to stronger antibody protection.

Still, the differences between those with and without symptoms were minor — everyone who received the shots had the intended immune response — but the study offered some reassurance about feeling unwell after vaccination.

"Those symptoms, though they may be unpleasant, may actually be working for you," Aric Prather, the study's lead author and a clinical psychologist at the University of California, San Francisco, told the The New York Times.

Researchers tracked the side effects of 363 adults who received two doses of the Pfizer or Moderna vaccines, giving them daily surveys for six days after they received their second shots. Measurements of skin temperature, heart rate and respiratory rate also were taken. At intervals of one month and six months after vaccination, researchers analyzed the presence of antibodies that fight the virus after vaccination.

The findings have not yet been reviewed for publication in a scientific journal, but they compare favorable to past studies that found a link between vaccine symptoms and stronger antibody responses.

One study published last year examined self-reported symptoms from 928 adults who received either the Pfizer or Moderna COVID-19 vaccines. They were split into three groups: those with multiple symptoms, those with symptoms only around the area of the shot, and those with no side effects.

Among the people in the study, 48% said they had multiple, systemic symptoms including fever and chills. Another 12% said they only felt symptoms near the injection, and 40% reported no side effects.

Nearly everyone in the study showed signs of a strong antibody response, but those who had multiple side effects showed slightly better immune responses.

“A lot of people have speculated over the years whether people who had more of a reaction to the vaccine might actually have that represent a more vigorous immune response. And these data would appear to support that," Dr. William Schaffner, a professor in the division of infectious diseases at Vanderbilt University Medical Center, told CNN. He was not involved in that study.

Studies on flu vaccines have shown a similar link between side effects and immune response. Flu vaccines and COVID-19 boosters are now often given together, which can lead to symptoms from one or both immunizations. 

Earlier this fall, Pfizer and Moderna each released new COVID-19 boosters. Although the new shots are recommended for everyone 6 months and older, booster shots have not been as widely sought in the U.S. as the initial vaccines were. One poll indicated about 75% of people skipped last year's booster.

About 4 million people in the U.S. got the new booster shots in September, according to the U.S. Department of Health & Human Services.

COVID-19 deaths and hospitalizations have declined over the last year, but the virus continues to evolve and has mutated into more than two dozen variants. As immunity from previous vaccination wanes, changes in the virus can make people who were previously vaccinated or infected more susceptible to new infections.

The new COVID-19 shots were reformulated to target the omicron subvariant XBB.1.5, which was dominant earlier this summer and closely related to the subvariants currently circulating.

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