Editor’s Note (4/18/23): On April 18 the Food and Drug Administration authorized a second bivalent COVID booster for people aged 65 and older who had their first bivalent shot at least four months earlier and for immunocompromised individuals who received it at least two months earlier. Additional doses can be given to immunocompromised people at their health care provider’s discretion.

Many people in the U.S. who are fully vaccinated and boosted for COVID have been waiting—eagerly in some cases—to receive another layer of protection as they pass the six-month mark after their last booster in fall 2022. But most will have to continue to wait. Late last month the Centers for Disease Control and Prevention upheld its existing COVID vaccination recommendations: the agency says that just one dose of the latest updated booster, often called the bivalent booster, is necessary for now. The Food and Drug Administration has also only authorized the same one-dose booster.

This contrasts with official guidance in other countries. In early March Canada and the U.K., for example, began offering an additional booster dose to certain populations at high risk of severe COVID, including elderly people, residents of long-term care facilities and immunocompromised individuals. The World Health Organization (WHO) recommends countries consider an additional booster six or 12 months after the last for older adults, those who have comorbidities or who are immunocompromised or pregnant, and frontline health care workers. The WHO also says healthy children from six months to 17 years old may not need any additional boosters.

“The messaging could get very confusing,” says William Schaffner, a preventive medicine and health policy professor at Vanderbilt University Medical Center, who is a consultant to the CDC’s Advisory Committee on Immunization Practices (ACIP). “One of the most difficult things I’ve learned during this pandemic is how hard it is for the general public to live with changing recommendations over time. They would like a definitive answer now.”

This uncertainty has created challenges for both the public and those in the health care field, including members of the ACIP. What do these recommendations mean for people—especially those at high risk of developing severe COVID—who want the extra layer of protection? Scientific American spoke to experts to find out what is and isn’t known about the immunity levels conferred by COVID boosters, what it means to get an additional booster outside the recommendations and what the situation might look like by this fall.

How do the current bivalent vaccination rates factor into the new guidance?

The bivalent booster now available in the U.S. came out in September 2022. It was formulated to cover the COVID-causing virus’s Omicron strains BA.4 and BA.5 and the original strains from 2020. After an initial wave of people received the booster in the fall and early winter, the rate of vaccinations has dropped.

A number of fully vaccinated and boosted people would like another booster for additional protection, Schaffner says, but a much larger population has not yet received any booster at all. Only 16.7 percent of the U.S. population (about 55 million people) have had the latest one—far fewer than officials had hoped. “That’s clearly been a source of considerable disappointment to everyone in public health,” Schaffner says. “The current public health thrust is not to give people an additional booster but to get people to take the first bivalent booster.”

The CDC says it continues to monitor emerging data but maintains its recommendation of one updated COVID vaccine for eligible people aged six months and older. “Too few people, particularly those who are older and at high risk for severe COVID-19, have taken advantage of getting an updated COVID-19 vaccine. And we encourage eligible individuals to speak with their health care provider and consider receiving one,” says CDC representative Kristen Nordlund.

Scientists in a vaccine working group within ACIP presented data in February that show vaccination uptake has declined after each official recommendation of an additional dose. The scientists pointed to several factors, including vaccine and COVID “fatigue” and a perception that initial vaccinations have provided enough immunity.

What do we know about the bivalent booster’s effectiveness and immunity levels?

The latest studies show the bivalent boosters effectively protect against severe disease and death. The CDC reported in February that mortality rates among people who received a bivalent booster were 14 times lower than in those who had never been vaccinated and three times lower than in people who received the original COVID vaccination series but no booster. Other early estimates also indicate the bivalent booster increases protection against two of the latest Omicron strains, XBB and XBB.1.5, for at least the first three months after vaccination in people who had previously received at least two of the past monovalent vaccine doses.

Notably, the bivalent shots are especially effective against COVID-associated hospitalization in older adults. But U.S. residents older than age 65 have been getting this booster at lower-than-expected rates—just 42 percent of that population has gotten it—says Carlos del Rio, a clinician and epidemiologist at Emory University. “If you’re telling me you’re 40 and you have no underlying conditions, it’s probably not an urgency to get another booster,” del Rio says. “But if you’re over 65, that’s a problem.”

Less is known about how long immunity lasts after receiving the bivalent vaccinations. ACIP’s COVID vaccine working group, however, says its information on the original monovalent series and boosters suggests protection against hospitalization starts waning four months after a person receives the dose. “It doesn’t go to zero,” Schaffner says. “After a period of months, it might go from 90 percent to 70 percent. So from a population basis, there still is substantial protection out there.”

On an individual basis, however, waning protection gets more complicated—especially in high-risk groups. Studies have demonstrated that monovalent COVID vaccine effectiveness in certain immunocompromised people, particularly organ or stem cell transplant recipients, is lower than in others. “Wouldn’t it be appropriate for [high-risk groups] to receive a spring or summer booster if it’s been five or six months since they have received the initial bivalent booster? It’s a perfectly reasonable question,” Schaffner says.

Protection appears to be restored after people receive additional doses over time, according to ACIP. “We are seeing very, very low rates—far less than 5 percent—of severe disease in people who are immunocompromised, well vaccinated and receive standard antiviral treatments,” said committee member Camille Kotton, an infectious disease clinician at Massachusetts General Hospital, during February’s ACIP meeting. She added, however, “There are still many, many immunocompromised [people] who have not taken the opportunity to get the bivalent vaccine.”

Can you get another booster if you want one?

Some members at the February ACIP meeting urged the FDA and CDC to allow flexibility with vaccine guidance and availability, especially for high-risk populations, as new data come out. ACIP member Michael Hogue, a pharmacy professor at Loma Linda University, suggested during the meeting that people speak with clinicians about this. “We want those clinicians to be able to make good decisions for the individual patient based upon their comfort and desire, as long as we have safety in mind. And it’s clear that we do in fact have a very safe vaccine with our bivalent vaccine,” Hogue said. “Flexibility just needs to be put into this, in some way, with both older adults and people with immunocompromising conditions.”

Under the latest recommendations, that flexibility doesn’t exist—technically. Some people have managed to “game the system” and get extra boosters, Schaffner says, noting that “there’s no vaccine police.”

To find a “friendly pharmacist” willing to provide that extra dose, “you may have to shop around a little bit. Many pharmacies and clinics will not [provide one] because they’re not authorized to do that,” he says. “That would be operating outside of the emergency use authorization under which the vaccine is currently being made available.”

People who get extra boosters do so at their own risk, and medical treatment for any side effects would not be covered by the government program authorized to provide benefits during the COVID public health emergency. “If you get a vaccine outside of the current guidelines, you are kind of out there on your own. And if there was a risk, albeit small, you are assuming it on your own,” Schaffner says. ACIP’s working group found that the longer the time between the two doses of the primary series, the lower the risk of myocarditis—heart tissue inflammation that has occurred in a small number of people who received COVID vaccines. But the risk of health problems from any of the COVID vaccines is very low.

Amira Roess, a global health and epidemiology professor at George Mason University, questions how the one-dose booster guidance could impact vaccine access for some people.

“What does that mean from a health equity perspective? Well, it means that you have to have a health care provider that you go to regularly—and we already know that there are lots of individuals in this country who don’t have access to high-quality health care,” Roess says. “If you can’t get the booster from [a local pharmacy], and you’re immune compromised, and you’re not seeing your health care provider regularly, you’re not going to have access to an additional booster, even if you knew you needed it.”

Access barriers persist among people experiencing homelessness, those with disabilities and some minority groups. CDC data show that as of April 5, only 9.2 percent of people who identify as Black and 8.8 percent of people who identify as Hispanic or Latino have received the latest booster.

“I can imagine that this gap in equity would be even larger should a second booster also be mandated,” says Jacinda Abdul-Mutakabbir, a clinical pharmacist and assistant professor at the University of California, San Diego. She adds that the end of the federal government’s official COVID public health emergency, set for May 11, will likely make equitable access even more difficult.

What does the future of COVID vaccinations look like?

So far all U.S. COVID vaccines and boosters have been government-supported and administered without charge. But along with subsidized treatment, testing and other services, this may change soon.

“This all gets complicated with the ending of the public health emergency, because that would mean vaccines are now going to cost you money,” del Rio says. “The government still has significant vaccines out there, but I think that’s going to be an issue going forward. I don’t know what the insurance plans are going to do.”

Schaffner and others say they have heard the FDA is considering an emergency use authorization for a spring or summer booster in high-risk groups. Several news reports also suggest such an announcement may be made in the coming weeks. When asked for comment, the FDA said it continues to monitor emerging data and will base any such decisions on new information.

“We hope that simplifying the COVID-19 vaccine regimen in the not too distant future will lead to the vaccination of more individuals in the coming years as we learn to live alongside SARS-CoV-2—potentially reducing serious outcomes, including hospitalization and death,” wrote an FDA spokesperson in an e-mail to Scientific American.

Schaffner says the current CDC and FDA guidance may signal a step away from rolling out COVID boosters every six months and toward an annual vaccination schedule. Many experts anticipate the next booster will be timed with the flu shots in the fall and will likely be an updated formula.

“Quite notably, there are still somewhere on the order of 250 to 300 COVID deaths daily in the United States,” Schaffner says. “So it’s by no means trivial.”