NEWS | January 27, 2021

Reactions, immunity, pregnancy and more: Answers for when you get the COVID-19 vaccine

Expect the strongest reactions after your second dose – and that means the vaccine is working

Dean Blumberg, chief of pediatric infectious diseases at UC Davis Children’s Hospital and Natascha Tuznik, assistant clinical professor of infectious diseases, answer questions about what to expect when you get the vaccine.

Reactions to the vaccine have been generally compared to a flu shot, though the second dose often brings a day or so of stronger reactions. Reactions to the vaccine have been generally compared to a flu shot, though the second dose often brings a day or so of stronger reactions.

What are the common reactions to the COVID-19 vaccines?

More than 20 million people have gotten at least one dose of their coronavirus vaccine, according to the Centers for Disease Control and Prevention (CDC) tracker, and the vast majority who talked about their side effects described them as similar to a flu shot.

They generally last a day or two at most.

“Even the strong reactions are much, much better than getting COVID-19,” Tuznik said. “It is not even close.”

The CDC says the most common effects are a sore arm and sometimes short-term fever, chills, tiredness and headaches. As reported in both studies, the majority of reactions beyond a sore arm happen after the second dose.

“Those are signs your body is building its immune response,” Blumberg said. “Everyone will have a slightly different reaction, but they don’t last long.”

What to know about having COVID-19 and getting a vaccine

More than 25 million Americans have had COVID-19 and people are getting sick at a rate of more than 150,000 per day in the U.S. Experts say almost all of them should still get a vaccine.

Could I get COVID-19 after I’ve been vaccinated?

It takes time for your body to build immunity, and you won’t reach full immunity until a week or two after your second shot. That’s why the CDC says it’s possible a person could still get sick with COVID-19, especially just before or just after the first shot.

The vaccines are 95% effective, meaning there is still a small chance you could get sick with COVID-19 even after reaching your full immunity, but it is also very likely your case would be much milder – which sometimes happens with flu vaccines – because your immune system will have a head start in fighting the disease.

“If you do get COVID-19 between doses of the vaccine, patients should wait until the acute illness is over and they’ve completed their isolation period,” Tuznik said. “It’s OK if the second dose is delayed in those cases.”

If I already had COVID-19, should I get a vaccine?

“Our recommendation is to get the vaccine, because we don’t know how long natural immunity from being sick will last,” Tuznik said. “We also recommend waiting until 90 days after all symptoms have disappeared.”

The reason for the wait: You will have some antibodies from the disease and are not likely to get COVID-19 again for at least 90 days, the CDC says. The waiting period will help reduce your reactions to the vaccine, plus it will let others who are more at risk get vaccinated first.

Can I get the vaccine while I have COVID-19?

The CDC says you must wait until you’ve recovered and finished the CDC recommended isolation time, which is 10 days after the symptoms first appear and 24 hours after a fever has disappeared. Then, just as with someone who had COVID-19 before getting vaccinated, Tuznik said it’s best to wait another 90 days before getting your shot.

Why is it common to have a stronger reaction after my second dose?

“That is normal,” Blumberg said. “After the second shot, your immune system really kicks into gear. That’s what causes the bigger reaction.”

It’s called a boosting phenomenon. It helps your immune system build an even greater ability to defeat COVID-19.

“When you get the first dose, your body learns to make antibodies to fight COVID-19,” said Tuznik.

The second shot launches them into action. “Your body says, ‘Oh look. A COVID particle. Attack!’” she said. “This more robust immune response and reaction is how you know it’s working.”

And although stronger reactions after the second dose are common, Blumberg said some people may have little or no reaction. He suggested planning for a strong reaction by not scheduling anything strenuous the day after your second dose.

If I don’t have any reaction to either shot, is the vaccine still working?

Yes, it is still working.

“Everyone reacts differently to every vaccine,” Tuznik said. “For some people, their reaction is no reaction. But your immune response is still building and the vaccine is still working. Don’t be discouraged.”

“The bottom line is, more than 20 million people have gotten at least one dose. There has been only a very small number of severe reactions.”
– Natascha Tuznik

What about severe reactions to the vaccine?

In a study released in January the CDC said the risk of anaphylaxis – a severe allergic reaction – is extremely low. Based on data from people who have received the first of the two recommended doses, only about one in every 90,000 people will experience anaphylaxis. The people most at risk are those who have had severe allergic reactions to other drugs or medications, foods and insect stings.

“The bottom line is, more than 20 million people have gotten at least one dose,” Tuznik said. “There has been only a very small number of severe reactions.” In contrast, more than one out of every 1,000 Americans has died from COVID-19.

Is there anything I should do before getting my COVID-19 vaccine?

There are only two cautions:

  • Don’t use any over-the-counter pain relievers before coming in for your shot.
  • If you are getting a flu shot or some other vaccination, do it at least 14 days before your first COVID-19 shot.

“After your vaccination, it’s OK to take pain relievers if you need them for a headache or mild fever or anything,” Tuznik said. “But don’t use them beforehand. We don’t yet know what impact that might have on your vaccine-induced antibody response.”

As for everything else, it’s just a normal day. “Eat what you normally eat, drink what you normally drink,” she said.

What should I do after I get my vaccine?

To be sure there are no serious reactions, you will have to wait 15 minutes before you leave (30 minutes if you have a history of strong allergic reactions). After that? “Just go about your day,” Tuznik said.

Maybe with a touch of restraint. Tuznik said it’s fine to have a glass of wine or any normal meal that evening. “But it’s not the time to overdo it,” she said. “Enjoy your day or evening in moderation.”

As for exercise later that day or the next, it’s OK to go for a run, walk or bike ride or to do anything you normally do, if you feel fine.

“Fatigue is a possible reaction, so just don’t push it,” she said. “If you’re not feeling great when you’re out there, just turn around and head for home.”

There is one caution: Wait at least 14 days after your second COVID-19 vaccine dose to get a flu shot – if you haven’t gotten one already – or any other vaccination.

Should I get a COVID-19 vaccine if I’m pregnant?

Experts recommend that most pregnant women get vaccinated but urge them to check with their doctors first. Although pregnant women were not included in the clinical trials, a small number of participants did get pregnant during the trials and the studies showed no adverse effects on the mothers or their babies.


Experts say you can exercise and go about your life after getting your shot, but fatigue can be a reaction so it’s wise to be moderate.

“It’s important to know that pregnant women are at increased risk of severe illness from COVID-19 – including death – compared with non-pregnant women of reproductive age,” Tuznik said.

When does COVID-19 immunity start?

This is not entirely clear. There does not appear to be any change in immunity in the first week after the first shot.

Data from the U.S. Food and Drug Administration (FDA) show that people begin to build some immunity – as much as 50% – 12 days after their first shot. But they won’t reach full immunity until seven days after their second Pfizer dose and 14 days after their second Moderna shot.

What happens if I only get one of the shots?

“I would strongly discourage people from getting just one dose,” Tuznik said. “You will definitely have diminished immunity, and it may not last.”

The vaccine makers have also said getting one dose is a bad idea, partly because it will create less immunity and because they have no evidence that any immunity would last long.

“(A stronger reaction after the second dose) is normal. After the second shot, your immune system really kicks into gear. That’s what causes the bigger reaction.”
– Dean Blumberg

If I wait longer than the recommended 21 or 28 days, will the vaccines still work?

The manufactures advised against delaying the second dose if at all possible, saying they have not studied whether the immunity would last.

However, the CDC’s latest guidelines say, “the second dose should be administered as close to the recommended interval as possible,” but if necessary, people are OK to go six weeks between the first and second dose.

Can I get my first dose from one vaccine manufacturer and my second dose from the other?

The CDC strongly recommends against that. The vaccines “are not interchangeable” and the impact of switching has not been studied, it says in its latest guidelines. However, the CDC says people may switch from one authorized vaccine in “exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available.”

How the vaccine was developed and tested.
Read more about the vaccines from UC Davis Health experts.
Learn more about patient vaccinations at UC Davis Health.