How long does it take COVID-19 vaccine to be effective after receiving shot?

FILE - In this Dec. 29, 2020, file photo, Penny Cracas, with the Chester County, Pa., Health Department, fills a syringe with the Moderna COVID-19 vaccine before administering it to emergency medical workers and healthcare personnel at the Chester County Government Services Center in West Chester, Pa. President Joe Bidens pledge that all U.S. adults will be eligible for coronavirus vaccines by May 1, 2021, is putting a new challenge in front of the nation. For that to happen, the federal government must deliver doses to hundreds of new vaccination sites and recruit more health care workers to administer the shots. (AP Photo/Matt Slocum, File) (Copyright 2020 The Associated Press. All rights reserved)

This is one of the top questions we receive right now:

How long after receiving a COVID-19 vaccine shot does it take for it to be effective?

The Centers for Disease Control and Prevention (CDC) says it typically takes a few weeks for the body to build immunity (protection against the virus that causes COVID-19) after vaccination.

“That means it’s possible a person could be infected with the virus that causes COVID-19 just before or just after vaccination and still get sick. This is because the vaccine has not had enough time to provide protection.” -- the CDC

Here’s a little more on how the vaccine becomes effective inside our bodies, from the CDC:

COVID-19 vaccines help our bodies develop immunity to the virus that causes COVID-19 without us having to get the illness. Different types of vaccines work in different ways to offer protection. But with all types of vaccines, the body is left with a supply of “memory” T-lymphocytes as well as B-lymphocytes that will remember how to fight that virus in the future.

It typically takes a few weeks after vaccination for the body to produce T-lymphocytes and B-lymphocytes. Therefore, it is possible that a person could be infected with the virus that causes COVID-19 just before or just after vaccination and then get sick because the vaccine did not have enough time to provide protection.

Sometimes after vaccination, the process of building immunity can cause symptoms, such as fever. These symptoms are normal and are signs that the body is building immunity.


Read more: Myths and Facts about COVID-19 Vaccines


More vaccine questions answered

Since the coronavirus pandemic began, Dr. Frank McGeorge has been keeping viewers up-to-date and informed on all fronts.

He’s been answering your questions about the vaccine, the vaccination process and more:

My husband and I were vaccinated and we received a bill afterward -- is that a mistake?

Yes. There have been cases of people being inadvertently billed after they received the vaccine, but those errors were identified and corrected. If you receive a bill for the vaccine administration, you should call and point out the error right away.

I had the two doses in December and January and I have been sick for four days now, throwing up, diarrhea headaches, chills, clammy, feel horrible -- can I still get the virus?

None of the vaccines are perfect and it is possible to become infected after being vaccinated, but it is unlikely.

If you have symptoms, you should get tested. Fortunately, the vaccines have been shown to be very good at preventing severe illness.

Has anyone who received the two vaccinations and waited the full two weeks for 95% protection tested positive for the virus?

Yes. In the study submitted for Moderna’s EUA, there were 11 people who received both doses of the vaccine and developed COVID-19, but there were 185 people who developed COVID in the placebo group. That’s where the 95% efficacy comes from.

Are the precautions after receiving the vaccine (and waiting two weeks) the same for a high risk person as for the general population?

Yes. The same precautions apply to everyone who is fully vaccinated.

My adult son is having ongoing chemotherapy, if we are both vaccinated, can we get together without wearing a mask?

Yes. The most recent CDC recommendations claim fully-vaccinated people are safe with each other, but if there are any extra concerns, you should encourage your son to talk to his doctor because depending on the chemotherapy, a mask may be important to protect against more than just coronavirus.

The CDC has stated that vaccinated Grandparents may now visit their grandchildren. Does this include non-vaccinated grandchildren attending high school or College? Aren’t they a problem, being out in the world also?

If you are fully vaccinated, the CDC believes it’s reasonably safe to hug a healthy unvaccinated person. That being said, if your college-age grandchild just got back from spring break, it wouldn’t hurt to wait a couple weeks for that hug.

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