With coronavirus variants popping up across the globe, new questions are beginning to arise about how long the immunity from the vaccines will last, and whether booster shots will be needed to maintain protection against the mutating virus.
Although vaccine companies are already in the process of conducting clinical trials for booster shots, and preparing for potential widespread distribution, a decision pertaining to if and when the updated shots will be needed in the months and years to come will ultimately be made by a team of independent scientists and U.S. government officials.
But in this once-in-a-lifetime pandemic, there is still very little known about COVID-19, and when a booster shot might be needed.
"With less than a year of real world data on the vaccines, we still don't know how long immunity will last and what the role boosters will play going forward," said Dr. John Brownstein, chief innovation officer at Boston Children's Hospital and an ABC News contributor.
The Centers for Disease Control and Prevention confirmed to ABC News that its independent group of scientists, the Advisory Committee on Immunization Processes, will meet and make a recommendation to CDC on booster shots, in the same way it has done with previously authorized coronavirus vaccines.
Conversations surrounding booster shots are not yet underway, according to one current ACIP member.
"The ACIP is dealing with a whole lot of other issues at the present time, and has not begun really a serious discussion of boosters," Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University in Nashville, told ABC News.
The Food and Drug Administration also told ABC News that it is prepared to address changes in the virus in anticipation of possible emerging variants. The agency has already updated its COVID-19 emergency use authorization guidance to provide recommendations to vaccine developers seeking to amend their EUA to address new variants.
"We want the American public to know that the FDA is using every tool in our toolbox to fight this pandemic, including pivoting as the virus adapts and we remain committed to getting these life-saving products to the frontlines," an FDA spokesperson said in a statement.
The three vaccines authorized by the FDA for emergency use have been proven to offer a high level of protection against the virus, to curtail the rate of virus transmission and to be largely effective against all known emerging variants.
Top medical experts and pharmaceutical executives have voiced their expectation that immunity will wane over time, rendering necessary a booster shot for vaccinated individuals, perhaps as early as this fall.
"Booster shots will be needed as we believe the virus is not going away," Moderna CEO Stephane Bancel said during a first-quarter earnings call to investors in early May.
Pfizer CEO Albert Bourla concurred, telling Axios last week that a booster shot will likely be needed "sooner than later."
"If they got their second shot eight months ago, they may need a third one," Bourla asserted, adding that booster shots could be coming between September and October of this year.
Medical experts interviewed by ABC News concurred that booster shots are likely, but not necessarily inevitable, at least for this fall.
"In my view, it is not yet clear whether we will need a booster shot this fall, winter, and if so, what that booster shot should be," Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center, told ABC News.
Health experts say that any recommendation about booster shots should be made by public health officials at the CDC and FDA, and not by pharmaceutical executives.
"Clearly, the pharmaceutical companies have an economic incentive, particularly the ones who have vaccines that are for profit," Barouch said. "We want the decision to be made based on public health solely and [not] on economic incentives of the companies."
The answer to whether or not we will need a booster depends on how quickly immunity fades following vaccination, and that answer is still unknown.
"Every month that goes by, we learn a little bit more -- that the protection persists, for how long the protection from the current vaccine lasts. It could last 12 months, 18 months, we don't know, and we'll only learn that with experience," Schaffner told ABC News.
Hence, the need for a booster shot will be based on evidence that the immunity from the vaccine is no longer providing sufficient protection to individuals, nor keeping transmission down to acceptable levels in terms of rates of infection, hospitalizations and deaths.
"If we start to see breakthrough infections of clinical significance, then booster doses would likely be recommended," Dr. Anna Durbin, professor of international health at the Johns Hopkins Bloomberg School of Public Health, explained.
Antibody titers, a blood test that determines the level of antibodies in the blood, could also be a clinical indicator of whether a booster is necessary. However, we should not be overly reliant on them as a metric, because antibodies are just one factor in our overall immunity.
According to Dr. Thaddeus Stappenbeck, chair of the Department of Inflammation and Immunity at Cleveland Clinic, antibody titers diminish over time, and this decline is "highly variable," particularly among older people.
In addition, potential outbreaks with new variants could become a key determining factor on booster vaccinations.
"When you start to see an increased rate of breakthrough infections or maybe even worse, start to see an increased number of people with symptomatic disease or people who are actually getting sick and have to go to hospital ... that's going to be a pretty clear indication that it's time for people to get a booster," said Dr. Angela Rasmussen, a virologist at the Georgetown Center for Global Health Science and Security.
Schaffner said it is important to assess the frequency of the variants among us.
"We wouldn't create a booster, I think, against the variant that appeared only occasionally, even if it did evade the protection of the vaccine," he said. "It would have to be a variant that looks like it's causing more illness, and as a consequence, more hospitalizations."
The surveillance for the variants, their frequency, and how they differ from the current vaccine protection will be an ongoing phenomenon.
"I think the decision around boosters needs to be at least somewhat tied to public health data. Unfortunately, part of the issue is that the decision about boosters has to be made ahead of knowing some of these answers. You're not going to want to wait for a surge and then decide you want to implement a booster, because that could be too late," said Brownstein.
Medical experts believe that it is becoming increasingly clear that the coronavirus will become endemic, much like influenza, and the COVID-19 boosters could conceivably be given at the same time as the influenza vaccine, in the late fall and winter.
"I think we're going to have to be super focused on the epidemiological surveillance and the impact of this virus on morbidity and mortality. I think we'll be able to draw from real world experiences in other countries and see how they handle outbreaks and waning population immunity," concluded Brownstein.
ABC News' Eric Strauss contributed to this report.