What happens if your second dose of COVID vaccine is delayed? Here’s what to know

Katie Camero
·4 min read

Both coronavirus vaccines authorized for emergency use in the U.S. require two doses separated by about one month to offer maximum protection against COVID-19.

As some states struggle with supply shortages, hesitant recipients and disorganization, the idea of delaying second doses has surfaced as one that could potentially save more lives in a shorter period of time. Postponements may also be inevitable, as supplies are limited nationwide.

But the desperate attempt to control the pandemic is controversial. Some health experts say the move is risky because the practice of postponing COVID-19 vaccine doses has not been tested, while others say our immune systems can handle delays up to months..

The Centers for Disease Control and Prevention said in a Thursday update that people should get their second dose “as close to the recommended 3-week or 1-month interval as possible,” but that you should not get it earlier than that.

The Pfizer-BioNTech vaccine requires a 21-day interval between the first and second dose, and the Moderna shot a 28-day period, “however, there is no maximum interval between the first and second doses for either vaccine,” the CDC said.

If it’s not possible to get the timing right, the agency said second doses for the Pfizer or Moderna jab can be delayed by up to six weeks after the first shot, although there’s limited data on how effective the vaccines are beyond that window.

Even if that’s not possible and your second shot is delayed further, the CDC says “there is no need to restart the series.”

Despite a lack of clinical trial evidence on vaccine efficacy outside the recommended dose interval, Dr. Buddy Creech, director of the Vanderbilt Vaccine Research Program at Vanderbilt University Medical Center in Tennessee, told NBC News delays shouldn’t be of concern.

“Your immune system is really smart,” Creech told the outlet. “It doesn’t forget what it saw the first time. Do not panic. Even if it’s four weeks, six weeks, eight weeks before you can get that second dose, it’s fine from an immune system standpoint.”

The U.S. Food and Drug Administration, the group responsible for determining if the COVID-19 vaccine candidates are safe, is against the idea completely.

“We know that some of these discussions about changing the dosing schedule or dose are based on a belief that changing the dose or dosing schedule can help get more vaccine to the public faster,” the FDA said in a Jan. 4 statement. “However, making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health.”

The administration cited “misinterpretation” of vaccine developers’ data on first-dose efficacy.

It said 98% and 92% of participants in the phase 3 clinical trials for the Pfizer-BioNTech and Moderna COVID-19 vaccines received two doses within the three- or four-week interval, respectively.

Those who only got one shot were “followed for a short period of time,” meaning the group cannot determine how much or how long protection lasts after a single dose.

“If people do not truly know how protective a vaccine is, there is the potential for harm because they may assume that they are fully protected when they are not, and accordingly, alter their behavior to take unnecessary risks,” the FDA said.

A separate team of researchers found that levels of antibodies— proteins that help fight foreign invaders such as viruses and bacteria — dropped about 15 to 21 days after a first dose with the Moderna vaccine.

By 28 days, the time at which a second dose is needed, antibody levels dropped significantly, according to the study on 34 healthy participants published Jan. 7 in the New England Journal of Medicine.

Health officials in the United Kingdom have decided to give second doses of COVID-19 vaccines 12 weeks after first doses in an effort to vaccinate more people sooner, a British Medical Journal blog reports.

Meanwhile, the German and U.S. governments have said they do not intend to delay shots.

Health experts agree, however, that getting two doses of a COVID-19 vaccine is a must to guarantee full protection, so a delayed second shot is still better than a single jab.