As the COVID-19 vaccines reach more people across the country, some people have asked: can we delay the second dose of Pfizer and Moderna vaccines to get more people vaccinated quickly? And how safe am I after my first dose?
As an immunologist, I hear this question frequently. The answer is that a single dose is very effective – but I want to add that you still need to get both doses. However, the issue is important, not only for your personal health, but also for the health of the country, as leaders are figuring out how to ensure there is enough vaccine for everyone who wants it.
Good news from abroad
A recent study in Israel showed that a single dose of the Pfizer COVID-19 vaccine is very effective, up to 85%.
The Sheba Medical Center reported its experience in vaccinating its nearly 10,000 staff members with the Pfizer COVID-19 vaccine. Vaccination there began on December 19, 2020, coinciding with the third wave of COVID-19 in Israel. The researchers looked at the decrease in SARS-CoV-2 infection and COVID-19 disease after vaccination. On January 24, 2021, 7,214 health workers received a first dose and 6,037 the second dose.
In total, there were 170 cases of infection between 19 December 2020 and 24 January 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 a re-analysis of the Phase 3 clinical trial data reported in the 2020 New England Journal of Medicine. The study included 52% protection against the first dose of infections that occurred in the first ten days after vaccination, when one would not expect the vaccine to have had time to generate protective antibodies.
Using data from the published study of the Pfizer vaccine, Public Health England determined that vaccine efficacy for 15-21 days after dose 1 was 89% – and before dose 2 on day 21. The range was between 52 % and 97%. For days 15-28, or until the first week after the second dose, the protection against the first dose is estimated at 91%. The range for this was between 74% and 97%. A second dose will not be expected to confer immunity within that time.
Bottom line
What do we know then? The Centers for Disease Control and Prevention calls on people to take both doses of Pfizer and Moderna vaccines. You should be assured that even after a single dose of one of the vaccines, you have very high levels of protection after your body has time to build up immunity, about a week. The scheduled second dose of these vaccines makes them even more effective, but at a time when the vaccine supply is limited, there is much to be said about the priority of the first dose for most people.
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This article was published from The Conversation, a non-profit news site dedicated to sharing ideas from academic experts. It was written by: William Petri, University of Virginia.
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William Petri receives funding from the National Institutes of Health, the Gates Foundation and Regeneron.