(CNN) The US Food and Drug Administration has expanded the emergency use authorization of the Pfizer and Moderna Covid-19 vaccines to allow adults 50 and older to get a second booster as early as four months after their first booster dose of any Covid-19 vaccine.
The move extends the availability of additional boosters to healthy older adults. The FDA had previously allowed additional shots for anyone 12 or older who was severely immune-deficient. This group of people can now receive a three-dose primary series and two boosters for a total of five doses.
The US Centers for Disease Control and Prevention followed suit, saying it is updating its vaccine pages to reflect the FDA's expanded eligibility.
"Current evidence suggests some waning of protection over time against serious outcomes from COVID-19 in older and immunocompromised individuals. Based on an analysis of emerging data, a second booster dose of either the Pfizer-BioNTech or Moderna COVID-19 vaccine could help increase protection levels for these higher-risk individuals," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, in a news release.
"Additionally, the data show that an initial booster dose is critical in helping to protect all adults from the potentially severe outcomes of COVID-19. So, those who have not received their initial booster dose are strongly encouraged to do so."
The FDA said that in making its decision, it had determined that the known and potential benefits of second boosters outweigh the known and potential risks for these populations.
The CDC also said in its statement that adults who got Johnson & Johnson's vaccine as their primary and first booster shots at least four months prior may now get an additional booster of Pfizer/BioNTech or Moderna's vaccines.
"Boosters are safe, and people over the age of 50 can now get an additional booster 4 months after their prior dose to increase their protection further," CDC Director Dr. Rochelle Walensky said in the statement. "This is especially important for those 65 and older and those 50 and older with underlying medical conditions that increase their risk for severe disease from COVID-19 as they are the most likely to benefit from receiving an additional booster dose at this time."
She said that both the CDC and the FDA "continue to evaluate the need for additional booster doses for all Americans."
The CDC's update is what's known as a permissive recommendation, a statement that the shots may be used in this age group for those who want them, reflecting a degree of uncertainty in the science. But it's not an official recommendation.
The lack of a clear endorsement for second boosters punts the work of weighing the risks and benefits of another vaccine dose to individuals, and it has caused no small amount of consternation for some vaccine advocates who say the marginal extra protection some may get won't be worth the confusion fourth doses create.
Dr. Megan Ranney, an emergency medicine physician who is the academic dean of Brown University's School of Public Health, says that throughout the pandemic, officials have been faced with making policy before they've had enough evidence to back it. Ranney sees this as another example.
She says it's not clear whether everybody needs a fourth dose right now, but having these approvals in place will provide flexibility to roll more boosters out quickly if they're needed.
"I see this approval from the Biden administration as being an insurance policy on their part," Ranney says. "It's a way to allow people to get the vaccine or the additional booster. But it also provides them with the flexibility so that should BA.2 be worse than we're expecting, they can then quickly roll it out. or God forbid, should there be another variant in the next couple of months that requires another booster they can quickly roll it out. So I'm reading it that way," Ranney said.
There is general scientific agreement that third doses help strengthen immunity against severe illness from the virus that causes Covid-19. But the science is far from settled on when, or even if, fourth doses might be needed since the vaccines continue to offer a high degree of protection against Covid-19 hospitalization and death, even as protection against illness wanes.
Much of the evidence examining the safety and effectiveness of a second booster shot comes from Israel, which has been recommending a fourth dose of coronavirus vaccine to adults 18 and older since the end of January.
The FDA said that in making its decision, it had reviewed data from Israel's Ministry of Health on more than 700,000 people 18 and up who had received second boosters of the Pfizer-BioNTech vaccine at least four months after their first booster. More than 600,000 of those people were over 60. The agency said that data had revealed no new safety problems tied to a fourth dose.
The FDA said data on the safety of the Moderna boosters, when used as a fourth dose, comes from a study of 120 people ages 18 and older who got a fourth dose of the vaccine at least four months after their third shot of the Pfizer vaccine. No new safety concerns were identified in the three weeks following the last dose.
Some of the data the FDA relied on to make its decision comes from previously published studies.
In a large study of more than half a million adults over the age of 60, those who received a second booster, or fourth dose, of a Covid-19 vaccine had 78% lower odds of death during the Omicron wave compared to those who had a third shot at least four months earlier. But the numbers of deaths were relatively low in both groups. After 40 days of follow up, there were 232 total deaths out of nearly 234,000 people who'd only had three doses of the Pfizer-BioNTech vaccine, compared with 92 deaths out of 328,000 people.
A smaller study of health care workers, which included younger adults, found that fourth boosters were safe and restored antibodies to the same levels reached after third doses. But fourth doses were only moderately effective -- about 30 to 40% -- at preventing illness. And most of the workers who got sick still had high viral loads, suggested that they were capable of transmitting the infection to others.
Additional studies from the UK show that the antibody boost from a booster dose wanes very quickly, within a matter of weeks. So some experts feel that considering available resources and the diminishing appetite to continue to get more and more boosters, that the United States should wait until there's a clear danger from a new wave of infections to roll out fourth doses. Some see the likely timing of that to be next fall.
"If you have only one bullet in your gun to shoot, I would prefer to hold fire until the fall, because that's when cases may really start to increase," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University.
Since January, Americans 12 and older have been eligible for a third vaccine dose, but only 46% of that demographic group has had a third shot.
Schaffner says he's worried that fourth doses will just confuse people who haven't decided to get a third dose.
"I'm very concerned about dividing and not being able to conquer because the messaging will get very, very confusing," he said, "And so I think public health officials and clinicians ought to be continuing to focus on getting the third dose into people who are eligible."