Pfizer, Moderna vaccines reduce COVID-19 infection risk by 90%

The COVID-19 vaccines developed by Pfizer Inc with BioNTech and Moderna are highly effective and reduced the risk of infection by 90% by two weeks after the second shot among healthcare personnel and first responders, according to a the U.S. study released on Monday.

The findings from a study by the U.S. Centers for Disease Control and Prevention of the real-world use of these messenger RNA (mRNA) vaccines confirm what was seen in large controlled clinical trials conducted before they received emergency use authorizations from the U.S. Food and Drug Administration.

Results showed that following the second dose of vaccine (the recommended number of doses), risk of infection was reduced by 90 percent two or more weeks after vaccination. Following a single dose of either vaccine, the participants’ risk of infection with SARS-CoV-2 was reduced by 80 percent two or more weeks after vaccination.

It takes about two weeks following each dose of vaccine for the body to produce antibodies that protect against infection. As a result, people are considered “partially vaccinated” two weeks after their first dose of mRNA vaccine and “fully vaccinated” two weeks after their second dose. These new vaccine effectiveness findings are consistent with those from Phase 3 clinical trials conducted with the vaccines before they received Emergency Use Authorizations from the Food and Drug Administration. Those clinical trials evaluated vaccine efficacy against COVID-19 disease, while this study evaluated vaccine effectiveness against infection, including infections that did not result in symptoms.

“This study shows that our national vaccination efforts are working. The authorized mRNA COVID-19 vaccines provided early, substantial real-world protection against infection for our nation’s health care personnel, first responders, and other frontline essential workers,” said CDC Director Rochelle P. Walensky, MD, MPH. “These findings should offer hope to the millions of Americans receiving COVID-19 vaccines each day and to those who will have the opportunity to roll up their sleeves and get vaccinated in the weeks ahead. The authorized vaccines are the key tool that will help bring an end to this devastating pandemic.”

One of this study’s strengths is its design: participants self-collected nasal swabs each week for RT-PCR laboratory testing, regardless of whether they had developed symptoms of illness. Researchers were able to look for evidence of SARS-CoV-2 infection irrespective of symptoms.

A small number (10.7 percent) of infections in this study were asymptomatic (i.e., did not result in symptoms).

However, the majority of infections (58 percent) occurred among people whose infections were identified by testing before they developed symptoms or knew they were infected.

The study demonstrated that these two mRNA vaccines can reduce the risk of all SARS-CoV-2 infections, not just symptomatic infections.