COVID Vaccine Dosing FAQs

Sandra Adamson Fryhofer, MD


January 25, 2021

This transcript has been edited for clarity.

This edition of Medicine Matters addresses mRNA vaccine dosing details that physicians need to know.

On December 11, 2020, Pfizer-BioNTech's mRNA COVID vaccine received FDA emergency use authorization for those aged 16 years and older. Two 30 µg (0.3 mL) doses of Pfizer's vaccine, given 21 days apart, are 95% effective in preventing COVID.

On December 18, 2020, FDA authorized emergency use of a second COVID vaccine, Moderna's mRNA vaccine, for those 18 and older. Two 100 µg (0.5 mL) doses of Moderna's vaccine, given 28 days apart, are 94.1% effective at preventing COVID.

Both are more than 90% effective. Vaccine efficacy after a two-dose series is 94%-95%. The second dose is not a booster; it's the second dose in the series. Patients need both doses for optimal protection. That's what's been studied.

You may have seen some recent articles and news reports suggesting ways to stretch vaccine supply and immunize more people. Proposals include reducing the number of doses, extending the time between doses, and using half a dose rather than a whole dose.

In fact, on January 5, 2021, three articles in the Annals of Internal Medicine (Tuite and colleagues, Paltiel and colleagues, Barnabas and colleagues) proposed different dosing schedules. On January 4, the FDA released a statement basically saying to stick with the science and what's been studied. Making these changes would be premature. "Using a single dose regimen or administering less than the doses studied in the clinical trials could affect duration of protection." And duration of protection, even with two full doses, is still unknown. More study is needed.

Acceptable dosing intervals have been a little confusing. CDC initially specified "a 4-day grace period" for giving doses, which created some confusion and misinterpretation. On January 6, CDC clarified and explained that the 4-day grace period is for giving the second dose early. There's no maximum interval between the doses for either vaccine. But for optimum protection, try to adhere to the respective 21-day (Pfizer) and 28-day (Moderna) dosing intervals.

New updates from CDC posted January 21 address guidance for real-life situations. With interruptions in vaccine supply due to erratic distribution, vaccinators need guidance on what to do when vaccine is not available when it's time to administer a second dose. In this less-than-ideal situation, CDC says the second dose of the Pfizer or Moderna vaccine can be given up to 6 weeks (42 days) after the first dose. However, the 28- and 21-day dosing intervals are still recommended.

What if the same vaccine product isn't available when it's time for the second dose?

mRNA COVID vaccine products are not interchangeable. Their ingredients are different. Every effort should be made to determine which vaccine product a patient received for the first dose, and the same product should be used for both doses.

On January 21, CDC clarified that in exceptional circumstances, when the first-dose vaccine product is not known, or the same product is not available, any available mRNA vaccine may be used for the second dose a minimum of 28 days and up to 42 days (6 weeks) after the first dose was administered. At present, if a patient has already received two doses using different mRNA vaccine products, no additional doses of either vaccine product should be administered.

CDC guidance on these issues may be updated as other vaccine products (protein subunit or viral vector vaccines) are authorized in the United States.

Can patients who have already had COVID get the vaccine?

Yes; it's safe and likely efficacious after COVID, but current evidence does suggest that reinfection is uncommon in the 90 days after initial infection. Patients don't need any testing before getting vaccinated.

Should patients who currently have COVID be vaccinated?

It is best to wait until they have recovered and are out of quarantine. Patients who received COVID monoclonal antibody or convalescent serum should wait at least 90 days so that this COVID treatment won't interfere with vaccine immune response.

When should patients who have had a known exposure to COVID and are still in quarantine be vaccinated?

These patients should wait until their quarantine is over before getting vaccinated.

Can COVID vaccine be given in combination with other vaccines?

CDC says try to allow a minimum 2-week window from other vaccine doses if you can.

When patients are vaccinated, they should be given a card with the date and type of vaccine product they received, and told to save this card. It is their proof of receipt of the vaccine. Also, they should make an appointment for their second dose as soon as possible.

Whoever administers a COVID vaccine should record the patient's vaccination in the appropriate immunization information system within 24 hours of vaccination. This is included in the CDC's COVID 19 Vaccination Program Provider Agreement that you have to sign in order to administer COVID vaccine.

One more important reminder: For now, even after vaccination, all vaccinated persons should still continue to wear masks and follow other recommendations to prevent COVID. We're still not sure how much — or if — mRNA vaccines reduce transmission or just how long protection lasts. Only time and more study will tell.

Vaccines are here. Now we have to get them into arms.

For Medicine Matters, I'm Dr Sandra Fryhofer.

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