It has been more than two months since the first COVID-19 vaccination was shot into an arm, and by far the most-asked reader questions are still about how people can make an appointment to be vaccinated and when they will be able to get vaccinated.

We’ve answered that more than once but have also received a number of reader questions about adverse reactions to the Moderna and Pfizer-BioNTech coronavirus vaccines, the two currently authorized for use in the United States.

This week’s FAQ Friday delves into what is known about the safety of the vaccines currently being used in the U.S. and what people who have had allergic reactions to other vaccines should know.

How safe are the COVID-19 vaccines?

The furious pace at which COVID-19 vaccines were developed, tested and approved for emergency use is remarkable. Before this pandemic, the fastest a vaccine had been developed was about four years, for the mumps.

Normally it can take as many as 15 to 20 years to make a vaccine, especially for a virus we haven’t before encountered, said Dr. Angela Shen, a visiting research scientist at the Vaccine Education Center at Children’s Hospital of Philadelphia.

“The fact that we have two vaccines authorized for use, a third one on appeal and a fourth and fifth kind of on the horizon, is a really big deal,” she said.


The rush to market forced the U.S. Food and Drug Administration (FDA) to be more flexible. For the emergency use requests, the FDA allowed vaccine makers to use two months of safety follow-up data and information from half of the people enrolled in the trials. Regular approval normally requires six months of safety follow-up as well as inspections of company manufacturing sites.

The clinical trials for the Pfizer-BioNTech and Moderna vaccines, which both require two shots to reach about 95% efficacy, proved they were safe, and the vaccines were granted FDA emergency use authorization late last year.

The Moderna and Pfizer-BioNTech vaccines are messenger RNA (mRNA) vaccines. Vaccines developed with this technology deliver pieces of SARS-CoV-2 mRNA containing instructions to make spike proteins, which human cells use to create copies.

On Wednesday, the Johnson & Johnson one-shot vaccine was endorsed by FDA scientists who said studies show that the COVID-19 vaccine prevents hospitalization and is effective against the disease.

A J&J study showed that moderate to severe cases of COVID-19 were reduced by 66% when examining cases that happened at least 28 days after vaccination.

The FDA’s Vaccines and Related Biological Products Advisory Committee will be discussing the J&J vaccine Friday and will make a decision about whether to grant it an emergency-use authorization.


The J&J vaccine isn’t an mRNA vaccine and instead uses a rare type of modified human cold virus as a vector that can’t cause sickness.

Should I be worried if I’ve had allergic reactions to other vaccines?

The U.S. Centers for Disease Control and Prevention (CDC) suggests that people who have had an allergic reaction immediately after receiving a vaccine or other injectable therapy for another disease, even if not severe, should speak with their doctor before getting the COVID-19 vaccination.

For people with allergies not related to vaccines, the CDC recommends they be vaccinated.

People who have had allergic reactions to polyethylene (PEG) or polysorbate shouldn’t get the vaccine, the CDC says. Polysorbate isn’t used in either Moderna’s or Pfizer-BioNTech’s vaccines but it is closely related to PEG.

The most recent data from the CDC shows having anaphylactic reactions — acute allergic reactions to an antigen — to the two mRNA vaccines is rare.

The CDC’s Jan. 29 Morbidity and Mortality Weekly Report says that between Dec. 21 and Jan. 10, there were 10 cases reported of anaphylaxis out of more than 4 million first doses of the Moderna vaccine, or 2.5 cases per 1 million doses. There were no deaths, and nine of the 10 cases of anaphylaxis happened within 15 minutes of vaccination.


The CDC told STAT News there were 45 confirmed cases of anaphylaxis, or 6.2 cases per 1 million doses, related to the Pfizer-BioNTech vaccine as of Jan. 19.

Anaphylaxis can be deadly but can also be quickly treated, which is why people are told to wait for at least 15 minutes at the vaccination site before leaving.

Information from The Associated Press was included in this story.

Do you have questions about the coronavirus that causes COVID-19?

Ask in the form below and we’ll dig for answers. If you’re using a mobile device and can’t see the form on this page, ask your question here. If you have specific medical questions, please contact your doctor.

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