COVID-19 vaccines prevent 90 percent of all infections, CDC study says

The two mRNA coronavirus vaccines from Moderna and Pfizer-BioNTech prevent 90 percent of infections, according to a new study from the Centers for Disease Control and Prevention (CDC).

The results show strong evidence that the vaccines work against preventing all infections, including asymptomatic and pre-symptomatic infections.

According to the CDC, both vaccines prevented 90 percent of infections two weeks after receiving the second of two doses.

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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. 

The study looked at 3,950 health care personnel, first responders and other essential workers and required them to self-collect nasal swabs each week for PCR lab testing for 13 straight weeks, regardless of whether they had developed symptoms of illness.

The CDC said it chose those groups because they are more likely than the general population to be exposed to the virus because of their occupations.

Self-swabbing once a week allowed researchers to look for evidence of coronavirus infection irrespective of symptoms. According to the CDC, only 10.7 percent of infections in the study were asymptomatic. 

However, the majority of infections occurred among people whose infections were identified by testing before they developed symptoms or knew they were infected, which is when they are most at risk of spreading the virus.

The CDC's findings are consistent with Pfizer's own study of Israeli data, which also found the vaccine to be highly effective against asymptomatic SARS-CoV-2 infections

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The findings are also consistent with the companies' respective phase 3 clinical trials conducted before the vaccines received emergency use authorizations from the Food and Drug Administration. 

However, those clinical trials showed strong efficacy against COVID-19 — meaning people did not become seriously ill, but it wasn't clear if the vaccines actually stopped a person from getting infected.

This study evaluated vaccine effectiveness against infection, including infections that did not result in symptoms.

Preventing both asymptomatic and pre-symptomatic infections among health care workers and other essential workers through vaccination can help prevent the spread of COVID-19 to those they care for or serve. 

“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,” CDC Director Rochelle WalenskyRochelle WalenskyCDC working to tighten testing requirement for international travelers Overnight Health Care — Presented by March of Dimes — Omicron sets off a flurry of responses CDC strengthens recommendation to say all adults should get booster shot MORE said in a statement.

“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,” Walensky said

The results will likely play into the ongoing debate about whether it would be safe to delay the second dose of the vaccines to give the first dose more quickly to more people. The Pfizer vaccine doses are meant to be given 21 days apart, while doses of Moderna's vaccine are meant to be given 28 days apart. 

The CDC acknowledged that the findings were consistent with other recent studies of partial vaccination following the first dose of Pfizer vaccine among health care providers.

Studies conducted in the United Kingdom and Israel showed that one dose was about 70 percent and 60 percent effective, respectively, against infection with the virus that causes COVID-19.  

The results confirm that people start to develop protection from the vaccine two weeks after their first dose, but the greatest protection was seen among those who had received both recommended doses of the vaccine and "reinforce" the CDC recommendation of two doses, the agency said.