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A new study looked at nearly 4,000 healthcare workers to see how effective vaccinatinos were. Dania Maxwell/Los Angeles Times via Getty Images
  • A new study shows that the Pfizer-BioNTech and Moderna-NIAID vaccines are effective at preventing infections in the real world.
  • The study was published March 29 in Morbidity and Mortality Weekly Report, a publication of the Centers for Disease Control and Prevention.
  • Researchers found that a two-dose vaccine regimen was 90 percent effective at preventing infections 2 weeks after receiving the second dose.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub and follow our live updates page for the most recent information on the COVID-19 pandemic.

A new study suggests that the COVID-19 vaccines developed by Pfizer-BioNTech and Moderna-NIAID are highly effective at preventing symptomatic and asymptomatic infections in real-world settings.

The study was published March 29 in Morbidity and Mortality Weekly Report, a publication of the Centers for Disease Control and Prevention (CDC).

Researchers found that a two-dose vaccine regimen was 90 percent effective at preventing infections 2 weeks after people received the second dose.

One dose was 80 percent effective 2 weeks after vaccination. This was based on a limited window between the first and second dose, so the study can’t show how effective one dose of the vaccine is long term.

These results are similar to those from earlier phase 3 clinical trials, which found an efficacy of more than 90 percent for both the Pfizer-BioNTech and Moderna-NIAID vaccines.

Efficacy is a measure of how well a vaccine works in the carefully controlled setting of a clinical trial.

Real-world effectiveness is sometimes lower due to a number of factors.

Dr. James H. Conway, a pediatric infectious disease specialist at the University of Wisconsin-Madison, said what we really care about is a vaccine’s effectiveness — its real-world potential.

With studies like this, “we’re starting to get a better sense of how powerful these vaccines are when people get out into the real world,” Conway said.

“So it should be extraordinarily reassuring to everybody that these vaccines work as well as we hoped they would,” he said.

The study included 3,950 healthcare workers, first responders, and other essential and frontline workers in eight U.S. locations. They received one of these vaccines between December 14, 2020, and March 13, 2021.

No participant had previously tested positive for SARS-CoV-2, the coronavirus that causes COVID-19.

Participants collected a weekly nasal swab, which they sent to a central laboratory for RT-PCR coronavirus testing.

They did this regardless of whether they were showing any symptoms of COVID-19. This allowed researchers to also identify people with asymptomatic or presymptomatic infections.

Around 10 percent of people with infections had no symptoms.

People also collected an additional nasal swab and saliva sample at the start of symptoms that might be due to COVID-19, such as fever, chills, cough, or shortness of breath.

During the study period, almost two-thirds of people received both doses, and around 12 percent received one dose. Both mRNA vaccines are given on a two-dose schedule.

Among unvaccinated people, 161 infections occurred during the study period. Three infections occurred in people who were fully immunized, and eight infections in people who were partially immunized.

Full immunization starts 14 days after the second dose of the vaccine. By then, most people have generated a strong immune response to the virus.

Partial immunization occurs at least 14 days after the first dose and before the second dose.

Researchers excluded infections that occurred fewer than 14 days after a person received their dose because it’s not clear what level of protection the vaccine offers during this time.

Based on this data, researchers estimated the vaccine’s real-world effectiveness to be very high, and showed that the vaccines work regardless of whether someone has symptoms.

“It’s pretty encouraging that [these vaccines] diminish symptomatic disease quite a bit, but they also make a major dent in the asymptomatic cases,” Conway said.

“Asymptomatic cases are where we’re most concerned,” he added, “because people may be shedding [virus particles] and not realize it.”

Effectiveness changed very little even when researchers took into account factors such as participants’ sex, age, ethnicity, and occupation.

However, they caution that because of the limited number of infections that occurred during the study period, the effectiveness levels should be viewed with some caution.

Researchers will continue gathering data on these study participants, which will allow them to more accurately estimate the vaccines’ effectiveness.

They also plan on genetically sequencing samples going forward to determine whether people contract infections with one of the new coronavirus variants.

Some variants are suspected of eluding the protection offered by COVID-19 vaccines.

Variants of concern were already circulating in the United States during the time of the study — and the vaccines still provided strong protection.

Regular genetic sequencing of participants’ samples, though, could enable researchers to see whether certain variants reduce the vaccines’ effectiveness.

The new research adds to a growing number of studies on the real-world effectiveness of COVID-19 vaccines.

“Excellent study indicating ~90 percent reduction in all infections (including asymptomatic) for Pfizer and Moderna vaccinees [sic] across sites in America,” Shane Crotty, a vaccine scientist at La Jolla Institute for Immunology, wrote on Twitter.

“Very consistent with earlier studies from the UK & Israel, and again consistent with these vaccines being good at stopping transmission,” he wrote.

None of the approved COVID-19 vaccines are completely effective against infection.

So, results from real-world studies like this can vary due to factors that increase or reduce a person’s risk for infection — such as how people behave once they’re vaccinated.

Conway said many people in this new study were healthcare workers and other frontline workers, so they likely continued to wear masks and physical distance even after they were vaccinated.

Even by themselves, nonmedical interventions like these can reduce the risk of infection. Combining them with vaccination protects people even more.

“When we think about protecting ourselves from the coronavirus and ending the pandemic, it’s not an either/or situation,” Conway said. “You will get the most bang for your buck by doing both.”

However, “once we get enough people vaccinated and reach herd immunity, that’s when we can back away from some of the mitigation activities that all of us have been doing.”