COVID-19 vaccine side effects and transmission: 4 Qs, answered

Vaccine small talk has trickled into Americans' daily conversations now that 33 percent of adults have received their first dose.

Many people are sharing anecdotes about their vaccination experience and side effects with friends and family, who then pass on what they've heard to others. Some of these experiences have evolved into theories about what side effects mean. At times, it can be difficult to know what are scientifically validated findings — even for healthcare workers.

Though scientists are still discovering new information, Becker's examined current research to shed more light on four common questions about vaccine side effects and virus transmission. Our findings are summarized below.

Do younger people have more reactions?

Yes. Young people tend to have a stronger physical response to the COVID-19 vaccine than older people, Sujan Shresta, PhD, a viral immunologist at La Jolla (Calif.) Institute for Immunology, told Scientific American

Data from the FDA Pfizer clinical vaccine trial backs up this idea. For participants ages 18 to 55,  88.7 percent reported at least one local reaction, such as injection site redness or pain, compared to 79.7 percent of people older than 55. Systemic adverse events including fatigue, headache and muscle pain were reported by 82.8 percent of younger participants, compared to 70.6 percent of individuals in the older age group. Fever was more common after the second dose and in the younger group (15.8 percent) compared to the older group (10.9 percent). 

Though older adults are reporting fewer side effects, it doesn't necessarily indicate how effective the vaccine is. Experts believe that may have more to do with the way the immune system ages, according to Scientific American.

An individual's immune system may have a heightened response to viruses similar to those it has previously encountered, whether recently or decades ago. "People with young kids who get exposed to seasonal coronaviruses quite a lot may actually have some cross-reactivity that gives them more side effects," said E. John Wherry, PhD, director of Philadelphia-based University of Pennsylvania's Institute for Immunology. 

If you have intense side effects from the vaccine, does that mean you've had COVID-19 before?

Maybe. A study published on medRxiv Feb. 1 and not yet peer reviewed suggests that people who have previously contracted COVID-19 have more intense side effects after receiving the first vaccine dose compared to those who haven't had COVID-19 before. The researchers analyzed data from 109 individuals receiving either Moderna's or Pfizer's vaccine, 41 of whom had had COVID-19 before. The researchers say those previously infected with COVID-19 were more likely to experience systemic side effects such as fatigue, headache, chills, fever and muscle or joint pain.  

On the other hand, Karen Landers, MD, area health officer for Alabama's public health department, said having had COVID-19 before shouldn't affect one's likelihood of experiencing side effects or not. In an interview with local NBC affiliate WAFF, Dr. Landers said she hasn't seen such findings in scientific literature. 

Do women experience more reactions to the vaccine than men?

Yes. More women have reported side effects from the vaccine than men, according to CDC data published in February. 

The agency analyzed safety data for more than 13.7 million vaccine doses administered during the first month of vaccinations in the U.S. During this time, the CDC received 6,994 reports of adverse events after vaccination. Nearly 91 percent of events were not serious, and 78.7 percent involved women. The most frequently reported symptoms were headache (22.4 percent), fatigue (16.5 percent) and dizziness (16.5 percent).

While women could simply be more likely to report their side effects, some health experts say hormone levels, genetic differences and vaccine dosing may play a role, reports The New York Times.. For example, sex hormones like estrogen can bind to immune cells and affect how they function, according to Sabra Klein, PhD, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore.

"I think that there is value to preparing women that they may experience more adverse reactions. That is normal, and likely reflective of their immune system working," she told the Times in early March. "This sex difference is completely consistent with past reports of other vaccines."

Do vaccinated people carry and spread the virus? 

Maybe. Emerging evidence suggests fully vaccinated people are less likely to have asymptomatic infections and could be less likely to transmit the virus to others. That said, the research is ongoing, and the CDC has not shared a definitive answer to this question yet.

"The risks of SARS-CoV-2 infection in fully vaccinated people cannot be completely eliminated as long as there is continued community transmission of the virus," the CDC said in an April 2 brief. "Vaccinated people could potentially still get COVID-19 and spread it to others."

The National Institutes of Health is funding a national study that will seek to answer whether vaccinated people can get infected asymptomatically and spread the virus to other people. The trial launched March 25 at 21 college campuses nationwide. Researchers will enroll 12,000 students who will perform daily nose swabs for four months. Half will receive Moderna's vaccine the day they enroll, and the other half will get vaccinated after the four-month period. Researchers will also identify 25,000 close contacts of study participants to assess the degree of transmission from vaccinated students. 

Trial results will be available by this fall, researchers said.


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