When Can More People Get a Second COVID Booster?

People who've only been eligible to get three doses of the COVID-19 vaccines—a primary series and one booster dose—will likely be eligible for a second booster shot this fall, according to public health experts.

Adults over 50 and immunocompromised individuals qualified for a second booster in March, but even as the Omicron subvariants ripped through the country causing a record amount of infections, health officials said that the general population still had sufficient protection against serious outcomes, and therefore didn't need to go out and get a fourth dose.

But as the virus evolved and gained the ability to evade the original version of the vaccines, vaccine manufacturers and public health officials have hinted that the next booster campaign featuring an updated vaccine targeting the latest circulating variants will roll out this fall—possibly as early as September.

"We expect that by early fall, this vaccine will be available and in pretty wide use for the population as we gear up for potentially another cycle of surges as the weather gets colder and winter approaches and folks head indoors," Jason Schwartz, PhD, an associate professor of public health and health policy at Yale School of Public Health, told Health.

Updated Bivalent Vaccines Expected This Fall for Americans

The vaccines that are expected to be administered in the upcoming booster campaign will protect against the original strain—which is what the current vaccine targets—and the Omicron subvariants, which have triggered the most recent waves of infections.

According to William Schaffner, MD, medical director of the National Foundation for Infectious Diseases and professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center, the updated vaccines were modified in the same way flu shots are updated each year.

"We look at the strains that are out there causing the disease and we add them to the vaccine," Dr. Schaffner told Health.

The new vaccines are technically called bivalent vaccines because they target two viral strains in the same shot.

Moderna's updated vaccine, announced in July, will target the original strain of COVID, as well as the BA.4 and BA.5 subvariants. The U.S. government has reportedly secured 66 million doses of its updated booster for a fall rollout, with an option to buy an additional 234 million doses.

Pfizer's updated vaccine is slightly different—it targets the original COVID strain and Omicron BA.1. The government has also reportedly purchased 105 million doses, which may include updated shots, with the option to secure another 195 million doses.

The updated Moderna vaccine recently approved as a booster in the United Kingdom is also a bivalent vaccine, but unlike the shot being developed for the U.S., it targets the original form of the virus from 2020 and the first version of Omicron, though it is said to work for BA.4 and BA.5 as well.

The fact that the UK authorized a new bivalent booster for adults ages 18 and up strongly suggests that the U.S. will soon follow suit, Dr. Schaffner said.

Though the U.S. bivalent vaccines are not yet available and we do not have solidified guidance for when and who will be eligible to get one, the FDA has previously said that booster doses will begin in fall 2022. And Dr. Schaffer said the medical community widely expects that most, if not all, adults will be recommended to receive the new vaccine this fall.

Most People Have Been Well Protected With Three Doses So Far

While people older than 50 and immunocompromised people were cleared for a second booster dose in March, the majority of Americans simply didn't need extra protection at that time.

"The thought was, at this point, a second booster earlier this summer [made] sense providing just a little bit of a top off; a little additional short-term protection for those individuals who are at the highest risk of severe outcomes," Dr. Schwartz said, adding that the benefit of revaccinating the entire U.S. population with the 2020 vaccine, at that time, was unlikely to provide a significant impact.

According to a Morbidity and Mortality Weekly Report published by the CDC in July, a third dose of an mRNA vaccine provided additional protection in immunocompetent adults.

When BA.1 was the predominant Omicron variant, two doses provided 61% vaccine effectiveness; three doses upped the effectiveness to 85%–92%. The same jump was seen with a third dose when BA.2 and BA.2.12.1 were dominant: two doses offered 24% protection, while a third dose boosted protection to 52%–69%.

Evidence has also shown that spacing out booster doses may provide stronger protection. "With increasing duration since the last dose, you often get a better response for the booster," Dr. Schaffner said.

He explained that, if you give another dose of a vaccine too soon to someone who may not necessarily need it yet—like immunocompetent people over the summer—you may be administering it when their protection is still relatively high. It's when that protection begins to wane, that a booster dose is most beneficial, he said.

There's also the issue of vaccine fatigue and confusion among many people. According to Dr. Schwartz, many public health officials believed it made the most sense to promote a second booster to more Americans with a new and improved product.

"It's really trying to meet the public where they are," Dr. Schwartz said. "[We're] trying not to overwhelm them with public health recommendations and waiting until we have a product that we think will really provide a substantial advance over just rebooting with the vaccine we had all this time."