How quickly will you get immunity once you get the COVID-19 vaccine? How it works and what you can expect

Medical Watch

The much hyped COVID-19 vaccines are set to be administered, but how soon do they give immunity?

So far the vaccine makers Pfizer, Moderna and Astra Zeneca, who have completed their trials and asked for approval around the world, offer their vaccine in two shots. But doctors said those who get it will have some protection after they roll up their sleeve the first time. 

Dr. Richard Novak is chief infectious diseases at UIC.

“Like all of these vaccines under development, it targets the spike protein of the virus,” he said.

In the United Kingdom people lined up for Pfizer’s vaccine, the first to get the nod from health regulators there. The inoculation showed a 95 percent efficacy in trials. The vaccine does not contain live virus. It has mRNA from the spike protein of SARS CoV-2.

“When it enters your cells, the cells read the mRNA of the protein which is expressed on the surface of the cell,” Novak said. “And then the immune system sees that and recognizes it doesn’t belong there and starts to make an immune response to it.”

The body recognizes the manufactured spike protein to learn how to react when the real thing takes hold and kick the immune system into overdrive.

But when that happens it can cause inflammation.

Vaccine recipients may begin to feel a response immediately including fever, headache, body aches and fatigue. But not everyone has a reaction.

“Certainly the majority of people who get the vaccine don’t feel anything,” Novak said.

Novak ran the Chicago arm of the Moderna trial and monitored patients after their injections. He said just because people don’t get side effects, their body is still working to search and destroy the novel coronavirus. And very rapidly, vaccine recipients have immunity.

“Even after the first dose it was working,” Novak said. “It starts right away. … It usually peaks at about two weeks after the injection.”


Depending on the vaccine manufacturer, at three or four weeks after the first shot, a second injection is administered. It’s a safeguard to make sure immunity lasts.

“You get a second injection a month out, or in the case of Pfizer is three weeks after the first one,” Novak said. “And that boosts the response even further, so you have even more antibodies. … We definitely know that the second shot boost the immune response. That’s a known fact so it’s more likely to last longer after you get both shots.”

Astra Zeneca’s shot made with Oxford University researchers was originally designed to be taken in one dose. But researchers decided two is better. Since we are facing a new illness, they wanted to take all precautions. In trials the efficacy was not as great for this vaccine, but doctors believe that’s because they switched gears mid-trial. Once two shots were given, it provided 90 percent efficacy for those in England’s trial. 

“The Astro Zeneca vaccine is a very important vaccine,” Novak said. “And that it is very easy to manufacture and very easy to store and ship and it’s very inexpensive.”

While not first to market, ease of distribution may lead this vaccine to be more widely used. Certainly it will be the choice for developing countries based on cost and refrigeration needs.

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