Nurse prepares a COVID vaccine dose.

Why are two doses needed for the COVID-19 vaccines?

After you get a COVID-19 vaccine, the nurse will give you a card that tells you which vaccine you received, either Pfizer-BioNTEch or Moderna.

On the back of the card, the nurse will write down the date you received the vaccine and the date of the appointment for your second dose.

But why do we need two doses of these vaccines? Isn’t one enough? Wouldn’t we be able to get more people vaccinated — and faster — if we all took just one dose?

That might sound reasonable, but full protection comes from receiving both doses.

The FDA is advising against any change in the dosing schedule or the number of doses, says Brahm Segal, MD, Chair Of Internal Medicine and Chief of Infectious Diseases at Roswell Park Comprehensive Cancer Center.

In the Pfizer-BioNTech vaccine trial, 98% of participants received their doses three weeks apart; 92% of participants in the Moderna trial received both doses within the recommended four-week interval. In both studies, two doses of the vaccine resulted in more than 90% protection from COVID-19. Although one dose of the vaccine will provide some protection against the virus, we don’t have enough information to judge how effective it will be or how long protection will last compared to the standard two doses, Dr. Segal said.

Further research might suggest a change in this practice, but right now it would be considered a risky move to modify the dosing guidelines established by the FDA and the vaccine manufacturers.

Timing of second dose key to making vaccine effective

It's important to receive your second dose on time: The second dose of the Pfizer-BioNTech vaccine should be administered three weeks (21 days) after the first, while the second dose of the Moderna vaccine should be administered four weeks (28 days) after the first. The FDA and CDC recently added that second doses can be administered within a grace period of four days earlier than the recommended date. Although the second dose should be administered as close to the recommended interval as possible, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be given up to six weeks (42 days) after the first dose.

You should still wear a mask and practice social distancing even after receiving both doses of the vaccine, Dr. Segal adds. Experts believe we will need to continue these protective measures until 70% of the population receives both doses of the vaccine.

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