More than 100 million people in the United States have taken one of the coronavirus vaccines authorized by the U.S. Food and Drug Administration, on track to more than meet President Biden’s goal of 200 million inoculations during his first 100 days in office.

But some people have not shown up for the second shot of the messenger-RNA vaccines, which require two doses to achieve the strongest and longest-lasting immunity.

Anthony Fauci, the nation’s leading infectious-diseases expert, recently expressed concern that although a single dose is proven to be up to 80 percent effective, “it is somewhat of a tenuous 80 percent.”

It is not yet known how long the antibodies from a single dose will last because that data was not included in the Pfizer and Moderna clinical trials. But health experts say it is clear that people who get a single shot and stop there will not get the full protective benefit of the vaccine.

“You’re in a tenuous zone if you don’t have the full impact,” Fauci told reporters.

More on the COVID-19 pandemic


What are the implications of skipping the second dose?

Federal and local health authorities have said a small number of people have failed to get their second doses of the Pfizer and Moderna vaccines, which were granted emergency use authorization last year. The Johnson & Johnson vaccine requires only one dose; however, federal health authorities have called for a pause in using the vaccine during an investigation into rare blood clots that have occurred at a rate of about 1 in 1.1 million vaccinations.

A recent study by the Centers for Disease Control and Prevention found that during the first couple of months of the coronavirus vaccination program, the vast majority of people who started the mRNA vaccine series finished it. However, about 3 percent of them did not.

Kristen Marks, an infectious-disease expert at Weill Cornell Medicine and New York-Presbyterian, said if people start skipping the second dose, she believes “you’d start to see more infections.”

“Most of the covid cases we’ve seen in vaccinated people that have landed in the hospital have been people who haven’t yet gotten the second dose. I think that’s telling us something,” she said.

The mRNA vaccines were designed to be given as a two-dose series based on clinical trial data, which showed that two doses of the Pfizer and Moderna vaccines were needed to reach maximum effectiveness of about 95%. In a real-world setting, one dose of the vaccines was shown to be 80% effective at preventing infection between the two doses in the series, according to a recent study from the CDC. But health experts say that those antibodies may never reach the intended level of protection and will most likely wane without the second shot.

Aside from vaccine effectiveness and longevity, John Moore, a professor of microbiology and immunology at Weill Cornell Medicine, said there is the issue of virus variants. Based on laboratory studies, he explained, researchers believe that some of the variants that are starting to circulate are capable of “essentially blowing past the antibody level that is induced by the first dose.”


The second dose is what “gives you a boost of antibodies,” providing protective immunity, he said.

Are there any situations in which I should do so?

Although it’s rare, the CDC said that anyone who experiences anaphylaxis — a severe allergic reaction — after the first dose, or even has an immediate allergic reaction that does not require emergency treatment, should not take the second dose of either mRNA vaccines.

But “just having a fever or body aches, that’s not a reason to not get the second dose,” Marks said.

There is also some debate about whether people who have had COVID-19 may be in a different category.

The CDC still recommends that people complete the vaccine series regardless of whether they have had COVID-19 because there are too many unknowns about how long they are protected. But there is some early research suggesting that some people who have had the virus achieve a stronger immune response from a single dose of the vaccines than those who have never been exposed.

Moore, at Weill Cornell Medicine, said there is increasing evidence that “one dose is sufficient to give a very strong immunity boost and that the second dose is not adding much” in some people who have had the virus. But, as the researchers point out, duration of these antibody responses will need further study.


It’s also important to remember that the immune response to COVID-19 varies from person to person, said Rob Murphy, executive director of the Institute for Global Health and a professor of medicine and biomedical engineering at Northwestern University. The level of natural antibodies in someone who has had the virus depends on whether the person had a mild case or a “clinically relevant” form that may have provided more protective immunity, he said.

“You just cannot predict the immunologic response you got from having covid. Some people probably won’t need a vaccine at all, but we don’t have the markers yet to prove that,” he said.

So, at least for now, the CDC has not changed its recommendations.

If I skipped the second dose, what should I do?

Experts agree — go get it now.

Technically, the second dose should be administered 21 days after the first dose for Pfizer and 28 days for Moderna. However, the CDC said in cases when it is not possible to get the second dose within that time frame, it can be administered up to six weeks after the first dose.

Even so, experts say people who have missed that window should still get the second dose — no matter how long it has been.

It is not recommended to start over with the vaccine series.

Moore said that there is simply too much evidence to ignore that the second dose is beneficial.

“You’re entitled to it. Go get it — that’s the message,” he said.