CDC study on efficacy of mRNA vaccines against symptomatic COVID-19

As the coronavirus disease 2019 (COVID-19) pandemic continued to wreak havoc on the global healthcare and economic scenarios, vaccine development became an area of great interest. The first severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines, from Pfizer/BioNTech and Moderna, were built on a messenger ribonucleic acid (mRNA) platform and received emergency use authorization (EUA) in the U.S. at the end of 2020.

A new study posted to the medRxiv* preprint server derives measures of substantial efficacy for these vaccines in preventing mild to moderate COVID-19 from a group of outpatients with and without a history of vaccination.


Vaccine efficacy (VE) was cited to be above 95% for both these vaccines against severe and symptomatic illness. The first groups to be targeted were the most vulnerable, namely, the elderly and frontline health workers. In phase two, vaccination was expanded to all those older than 16 years. However, there is little data on how effectively these vaccines prevent COVID-19 in a group of individuals with COVID-19-like symptoms short of requiring hospital admission.

The importance of this information is threefold. Firstly, it helps understand the degree of protection among people with mild or moderate symptoms; it helps reduce the load on health services due to supposed COVID-19; and it will help frame community measures to prevent virus spread during the ongoing vaccination campaign.

How was the study done?

The researchers retrieved vaccination and clinical data from the United States Influenza Vaccine Effectiveness Network (U.S. Flu VE Network), which monitors the VE of flu shots every year.

The study period lasted from February 1 to May 28, 2021, with over 800 participants presenting with COVID-like illness, all at least 16 years or more. Of these, almost 30% returned a positive test for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Risk factors for test positivity included male sex, non-Hispanic Black ethnicity, age below 65 years, and being tested in Michigan or Pennsylvania. The peak of test positivity was during the second week of April.

Of the ~800 participants in the study, 364 were vaccine recipients; over 60% had taken the Pfizer vaccine, and the rest the Moderna vaccine. There were 37 SARS-CoV-2-positive cases in this group.

When compared with the test results, about one in seven SARS-CoV-2-positive cases had taken one or more doses of an mRNA vaccine. Less than half (46%) of the vaccinated cases had taken two doses of the vaccine, with 15/37 having taken the Pfizer vaccine and 2 the Moderna vaccine.

Of the remaining 20, 15 had received a single dose of the Pfizer vaccine and 5 the Moderna vaccine. In contrast, 57% of non-infected vaccinated individuals (n=327) had been vaccinated. Of these, 231 and 96 were partly or fully vaccinated, respectively.

The VE in outpatient CLI was, therefore, 91% and 75% among the fully and partly vaccinated.

What are the implications?

This study shows that the mRNA vaccines reduce symptomatic confirmed COVID-19 by 91% in the fully vaccinated group, vs. 75% among the partially vaccinated. These corroborate and strengthen the evidence from earlier clinical trials that these vaccines reduce the severity of illness across the spectrum of COVID-19.

The setting of the current study, which focused on outpatients, is suitable for detecting mild infections and infections in younger patients, who are being affected in larger proportions as the pandemic progresses, and who are more likely to develop moderate rather than severe symptoms.

Secondly, this group is more likely to be unvaccinated, predisposing them to a higher risk of disease. Thirdly, outpatient studies are geared towards patients with mild and moderate disease. These factors ensure that vaccine efficacy against SARS-CoV-2 in real-time, especially variants of concern, can be better monitored in this setting, which precludes many sources of confounding.

In this study, receipt of mRNA vaccines was associated with prevention of most mild to moderate COVID-19 in outpatients seeking medical care or testing in the U.S. The findings support ACIP recommendations to vaccinate eligible persons as well as efforts to increase vaccine coverage in the U.S. population for the prevention of symptomatic illness.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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