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VERIFY: What happens if I get the 1st dose of the COVID-19 vaccine but delay the 2nd?

The second coronavirus vaccine dose is about building memory within your body, so there isn't a 'waiting too long' period as recognized by the CDC.

The most common COVID-19 vaccines are produced by Pfizer and Moderna right now. Both are variations of an mRNA vaccine that has to be delivered in two separate doses to be fully effective. 

But three weeks ago the UK announced that it would be spacing out the second dose by up to 12 weeks in order to give the first shot to more people. And there are now reports that the first dose could possibly be halved to make the available supply go further.  


What happens if you can only get half the coronavirus vaccine due to a spread out delivery time or because someone halves the dose?


According to Dr. Kawsar Talaat, an infectious disease expert and vaccine researcher at Johns Hopkins, there’s little risk in “waiting too long,” because the first dose of the vaccine causes the body to start building immunity.

The Centers for Disease Control and Prevention recommends that the second dose of vaccines be administered within three weeks to one month, but stated that “there is no maximum interval between the first and second doses for either vaccine.”


The VERIFY team received multiple viewer questions requesting clarification on what will happen if they can only get the first dose of a COVID-19 vaccine or if both their doses are halved. 


Dr. Kawsar Talaat - Infectious Disease Physician and vaccine researcher at the Center for Immunization Research at Johns Hopkins.

Centers for Disease Control and Prevention - Interim Clinical Considerations for Use of mRNA COVID-19 Vaccines Currently Authorized in the United States

Food and Drug Administration - FDA Statement on Following the Authorized Dosing Schedules for COVID-19 Vaccines

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According to Talaat, clinical trials show that there’s no decrease in the effectiveness of the vaccine if the person has to wait longer for the second dose. She said any dose is important because it “give[s] your body a chance to develop an immune response and some early immunity memory.”

The second dose of the virus then builds on that immunity. Talaat said that waiting longer for the second dose could actually lead to a stronger immune boost. 

“It's that memory response that you're boosting with the second dose,” she said. “So waiting a little bit longer probably gives you a better boost.” 

According to the CDC, “there is no maximum interval between the first and second doses for either vaccine.” They add that if the second dose is administered outside the recommended window, “there is no need to restart the series.”

That means that while it may be ideal to get a second dose in the recommended window to give your body the highest level of immunity possible, delaying that second dose is unlikely to make the vaccine less effective. 

So what about halving the first dose? That likely would cause the vaccine to be less effective. 

Earlier this month, the FDA released a statement clarifying that halving the first dose as an attempt to vaccinate more people could potentially create more harm as “making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health." 

Talaat said that if authorities or healthcare providers want to spare doses, she recommends giving people the first dose and wait for the second dose to be fully available, saying, “I'm sure that within a couple of months, we'll have much more vaccine supply. So to wait on the availability of the second dose would be better than giving people a fraction of a dose, because I don't think we have enough data for that.”

When it comes to immunity vs. transmission, Talaat clarified that while the COVID-19 vaccine provides immunity, there’s no data about its efficacy against transmission. “It is important to continue to wear your mask, continue to wash your hands and social distancing, as we need to  be careful until we have enough people vaccinated in our community. That’s when the spread of the virus will decrease, and we’re still a long way from that," she said.

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