The conversation
How effective is the first injection of the Pfizer or Moderna vaccine?
UPS employees distribute the first doses of the Pfizer vaccine at the UPS hub in Louisville, Ky., On December 13, 2020. Michael Clevenger / Getty ImagesHow COVID-19 vaccines reach more people across the country, some people ask: Will that it is possible to delay the second dose of Pfizer and Moderna vaccines to allow more people to be vaccinated more quickly? And how safe am I after my first dose? As an immunologist, I often hear this question. The answer is that a single dose is very effective – but I would add that you should still receive both doses. The issue is important, however, not only for your personal health, but also for the health of the country, as leaders discover how to ensure that there is enough vaccine for everyone who wants it. Medical workers vaccinate members of the medical team against COVID-19 on December 20, 2020 in Tel Aviv. Amir Levy / Getty Images Good news from abroad A recent study in Israel showed that a single dose of the Pfizer COVID-19 vaccine is highly effective, up to 85%. The Sheba Medical Center reported its experience with vaccinating its nearly 10,000 employees with the Pfizer COVID-19 vaccine. Vaccination started on December 19, 2020, which coincided with the third wave of COVID-19 in Israel. The researchers looked at the rate of reduction of SARS-CoV-2 infection and COVID-19 disease after vaccination. On January 24, 2021, 7,214 health professionals received the first dose and 6,037 received the second. In all, 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 a reanalysis of data from phase 3 clinical trials reported in 2020 in the New England Journal of Medicine. In that study, the protection of 52% of the first dose included infections that occurred in the first 10 days after vaccination, when the vaccine would not have been expected to have had time to generate protective anti-spike antibodies. Using data from the published study of the Pfizer vaccine, Public Health England determined that the vaccine’s effectiveness was 89% for 15-21 days after dose 1 – 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, protection against the first dose was estimated at 91%. The range for this was between 74% and 97%. A second dose is not expected to confer immunity within this period. Conclusion So, what do we know? The Centers for Disease Control and Prevention urge people to take both doses of the Pfizer and Moderna vaccines. You should be sure that even after a single dose of any of these vaccines, you will have very high levels of protection after your body has time to build up immunity, about a week. The second scheduled 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 how to prioritize the first dose for most people. [Deep knowledge, daily. Sign up for The Conversation’s newsletter.]This article was republished from The Conversation, a non-profit news site dedicated to sharing ideas from academic experts. It was written by: William Petri, University of Virginia. Read more: Delaying the second doses of the COVID-19 vaccine will make supplies last longer, but carries risks. Most vaccine skeptics plan to refuse a COVID-19 vaccine, a study suggests, and that could be a major problem. William Petri receives funding from the National Institutes of Health, the Gates Foundation and Regeneron.