How effective is the first shot of the Pfizer or Moderna vaccine?


AIf COVID-19 vaccines reach more people across the country, some people have asked: Could we delay the second dose of Pfizer and Moderna vaccines to allow more people to be vaccinated faster? And, how safe am I after the first dose?

As a immunologist, I hear this question frequently. The answer is that a single dose is very effective, but I would add that you should still take both doses. The issue is important, however, not only for your personal health, but also for the health of the country, as leaders figure out how to make sure there is enough vaccine for everyone who wants it.

Good news from abroad

A recent study in Israel showed that there is a single dose of the Pfizer COVID-19 vaccine highly effective, up to 85%.

Sheba Medical Center reported its experience with vaccinating its nearly 10,000 staff members with the Pfizer COVID-19 vaccine. Vaccination there began on December 19, 2020, which coincided with the third wave of COVID-19 in Israel. The researchers looked to see the rate of reduction of SARS-CoV-2 infection and COVID-19 disease after vaccination. As of January 24, 2021, 7,214 health workers had received a first dose and 6,037 had received a second dose.

In total, there were 170 cases of infection between December 19, 2020 and January 24, 2021. Of these, 89 people, or 52%, were not vaccinated; 78 people, or 46%, tested positive after the first dose; and three, or 2%, tested positive after the second dose.

This is consistent with one reanalysis of data from phase 3 clinical trials published in 2020 in the New England Journal of Medicine. In this study, 52% protection from the first dose included infections that occurred in the first ten days after vaccination, when the vaccine was not expected to have had time to generate protective anti-bite antibodies.

Using data from the published study of the Pfizer vaccine, Public Health England determined it the effectiveness of the vaccine was 89% for 15-21 days after dose 1 – and before dose 2 on day 21. The range was between 52% and 97%. During days 15-28, or until the first week after the second dose, the protection of the first dose was estimated at 91%. The range was between 74% and 97%. A second dose was not expected to confer immunity within this period.

Bottom line

What do we know, then? The Centers for Disease Control and Prevention urges people to get both doses of the Pfizer and Moderna vaccines. You should make sure that even after a single dose of any of these vaccines, you have very high levels of protection after your body has time to build immunity, about a week. The scheduled second dose of these vaccines makes them even more effective, but at a time when vaccine supply is limited, there is much to be said about prioritizing the first dose for most people.

William Petri, Professor of Medicine, University of Virginia

This article is republished from The conversation under a Creative Commons license. Read the original article.

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