About 1 in 7 Who Get Pfizer Vaccine Will Have Any 'Systemic' Side Effect: Study

WEDNESDAY, April 28, 2021 (HealthDay News) -- Only 13.5% of people will suffer systemic side effects after getting Pfizer's coronavirus vaccine, a new study finds.

These systemic symptoms include headache, fatigue and tenderness. Most side effects are mild and peak in the day after being vaccinated, and they only last for one to two days, according to the findings.

The analysis was done by researchers from King's College London using data on more than 627,000 people. They found fewer side effects with both the Pfizer and AstraZeneca vaccines than was seen in clinical trials. AstraZeneca's vaccine has not been approved for use in the United States.

Study author Dr. Cristina Menni said, "Our results support the aftereffects safety of both vaccines with fewer side effects in the general population than reported in the Pfizer and AstraZeneca experimental trials and should help allay safety concerns of people willing to get vaccinated."

The investigators also found lower infection rates in the 12 to 21 days after the first dose of the Pfizer (58%) and AstraZeneca (39%) vaccines, compared with people who weren't vaccinated. The drop in infection rates at least 21 days after the first dose of Pfizer was 69%, while it was 60% for AstraZeneca.

This large analysis looked at side effects including headache, fatigue, chills and shivering, diarrhea, fever, joint pain, muscle pain and nausea. They also looked at side effects where the injection took place in the arm, including pain at the injection site, swelling, tenderness, redness, itch, warmth and swollen armpit glands.

Other findings include:

• Nearly 14% had side effects after the first Pfizer dose, 22% after the second Pfizer dose and 34% after the first AstraZeneca dose. The most common side effect was headache. Eight percent of people reported suffered headaches after the first Pfizer dose and 13% after the second Pfizer dose.
• About a quarter (23%) of those who had the first dose of the AstraZeneca vaccine reported a headache.
• The second most common side effect was fatigue. Eight percent and 14% of participants reported fatigue after the first and second dose of Pfizer vaccine, while 21% reported fatigue after their first dose of AstraZeneca vaccine. The most common local side effect was tenderness: 57% and 51% after the first and second dose of Pfizer vaccine, and 49% after the first dose of AstraZeneca vaccine.
• Side effects were more common among people under age 55 and women.
• People who had COVID-19 were three times more likely to have side effects after doses of the Pfizer vaccine than those who didn't have COVID-19. For those who had the AstraZeneca vaccine, that effect was nearly two times greater.
• People who had COVID-19 were also more likely to have local side effects regardless of which vaccine they got.

"The data should reassure many people that in the real world, aftereffects of the vaccine are usually mild and short-lived, especially in the over 50s who are most at risk of the infection," said lead scientist Tim Spector, a professor of genetic epidemiology at King's College London.

"The results also show up to 70% protection after three weeks following a single dose, which is fantastic news for the country, especially as more people have now had their second jabs," Spector said in a college news release.

The report was published April 27 in The Lancet Infectious Diseases journal.

More information

For more on COVID-19 vaccines, head to the U.S. Centers for Disease Control and Prevention.

SOURCE: King's College London, news release, April 27, 2021