Do Pfizer and Moderna vaccines prevent SARS-CoV-2 infection? Can you transmit the virus after you’ve been vaccinated? How effective are mRNA vaccines in real world scenarios?

Randomized placebo-controlled Phase III clinical trials, have demonstrated that mRNA vaccines are great than 90% effective at preventing symptomatic COVID-19 and especially effective at preventing severe disease. But whether vaccination prevents SARS-CoV-2 infection in real world conditions has yet to be determined.

In other words, we have known for awhile that COVID-19 vaccines are highly effective at preventing people from getting sick and requiring hospitalization or dying from COVID-19, but we have not known whether vaccination prevents the virus from infecting people altogether or if it’s possible for people to still acquire asymptomatic SARS-CoV-2 infection and unknowingly spread the virus after being vaccinated.

On March 29, 2021, the CDC’s Morbidity and Mortality Weekly Report published data on the subject.
From December 14, 2020- March 13, 2021, weekly SARS-CoV-2 testing was conducted on a cohort of 3,950 frontline workers, regardless of symptom status. Only individuals with no documented history of SARS-CoV-2 infection prior to enrollment were qualified to participate in the study.

The final cohort consisted of:
• Healthcare Personnel - physicians, physician assistants, nurse practitioners, dentists, nurses, therapists, technicians, medical assistants, orderlies and all other persons providing clinical support in inpatient or outpatient settings.
• First Responders- firefighters, law enforcement, corrections and emergency medical technicians.
• Other Essential and Frontline Workers - those in hospitality, delivery, and retail; teachers; and all other occupations that require contact within 3 feet of the public, customers, or coworkers as a routine part of their job.

75% of participants received one or more doses of vaccine during the study. 2,479 or 62.8% received both doses of mRNA vaccine, while 477 or 12.1% had received only one dose of mRNA vaccine by the end of the study period.

Vaccination status was confirmed through self-report, electronic medical records and direct upload of vaccine card images at 8 U.S. locations. Which vaccine was administered varied based on time and availability of each product. 62.7% of vaccinated individuals received the Pfizer vaccine and 29.6% received Moderna. The remaining 7.7% were still pending product verification at the time of publication.

There were two major components to the design of this study. Active surveillance for COVID-19-related symptoms and weekly PCR testing for SARS-CoV-2 infection.

For 13 consecutive weeks participants were asked to keep track of and report any fever, chills, cough, shortness of breath, sore throat, diarrhea, muscle aches or loss of smell or taste that they experienced.

They were also asked to self-collect weekly midturbinate nasal swabs and ship them on cold packs to the Marshfield Clinic Laboratory in Marshfield, Wisconsin, where all samples were tested for the presence of SARS-CoV-2 RNA via RT-PCR, regardless of whether or not the participant was exhibiting symptoms. In the event that COVID-19-related symptoms did develop, an additional nasal swab and saliva specimen were collected at the time of symptom onset and tested in the same Marshfield Clinic Laboratory, as well.

Over the course of this study, 205 participants in total were diagnosed with SARS-CoV-2 infection by RT-PCR. 87.3% of these correlated with cases that displayed symptoms of COVID-19-associated illness, 2.0% with cases that displayed symptoms that were inconsistent with this study’s definition of COVID-19-associated illness, and 10.7% reported no symptoms at all. Two hospitalizations and zero deaths occurred.

Individuals were considered partially immunized 14 days or more after the first dose of vaccine and fully immunized 14 days or more after the second. Immunity was considered indeterminate during the 13-person days in between partial or full immunization.

161 infections of SARS-Cov-2 were confirmed by RT-PCR during the 116,657 person-days of this study when participants were unvaccinated.

33 infections were confirmed during the indeterminate period and were excluded from analysis.
8 infections were confirmed during the 41,856 person-days when individuals were partially immunized. And 3 infections occurred during the 78,902 person-days when participants were fully vaccinated.

The incidence rate for partially vaccinated individuals was 0.19 per 1,000 person days, and 0.04 per 1,000 person days for fully vaccinated individuals. Estimated adjusted vaccine effectiveness was therefore 80% in partially vaccinated individuals and 90% in fully vaccinated individuals.

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