A healthcare professional puts a bandage on the arm of an older woman after she received a vaccinationShare on Pinterest
The new COVID-19 booster shots are being strongly recommended for people 65 years and older. Luis Alvarez/Getty Images

In late August, the Food and Drug Administration (FDA) authorized the use of a bivalent COVID-19 booster that targets the Omicron variant.

The new shot contains both the original formulation as well as a component that zeroes in on the Omicron BA.4 and BA.5 variants.

It’s the first updated COVID-19 vaccine to be authorized in the United States and it was given the go-ahead despite the fact it hasn’t been tested yet on humans. FDA officials said they were confident in the booster’s effectiveness and safety because of the previous testing done on COVID-19 vaccines.

Both the Pfizer and Moderna boosters will be available at pharmacies and other locations in the coming weeks.

Will the new shots be effective? Who should get them? And when is the best time to get the booster?

Healthline put these questions to two highly regarded experts on infectious diseases.

Dr. Monica Gandhi, MPH, is a professor of medicine at the University of California San Francisco.

Dr. William Schaffner, is a professor of preventive medicine at Vanderbilt University in Tennessee.

Schaffner: The new updated COVID vaccines are available for persons who have completed a primary vaccination series and at least two months have elapsed since their last dose of COVID vaccine (from either the primary series or previous boosters). The Moderna updated vaccine is available to persons 18 years of age and older; the Pfizer vaccine for persons age 12 and older. If you have recently recovered from COVID infection, you should wait at least 3 months before receiving the new updated vaccine.

Gandhi: There is some recent data that can help refine how we use these boosters. A recent paper in JAMA shows a very strong protective effect of a single booster against severe disease with BA1 and BA2 that continues to demonstrate the powerful cellular immunity (T and B cells) triggered by the vaccines. Another recent paper in JAMA shows that a single booster with an mRNA vaccine provides additional protection for at least 6 months so that a second booster or 4th shot should not be required for older individuals during this time. A third paper tells us who is still at risk of severe breakthroughs during Omicron and requires Paxlovid to prevent hospitalizations and that is individuals 65 and older. Therefore, given all of the data showing strong protection of the previous mRNA vaccines against severe disease, I would recommend this Omicron specific booster for older people (65 and older) and immunocompromised who benefit from an “antibody boost” during times of high viral circulation. This is because B cells typically take 2-4 days to make neutralizing antibodies, which may be too long to wait for those who are more susceptible to severe disease. At some point, we need to clarify the goals of our booster strategy; if this is to prevent severe disease (like in other countries), we likely will only be giving regular boosters to older and immunocompromised individuals only.

Gandhi: If you were recently infected and were vaccinated before, you have a very powerful form of immunity called “hybrid immunity” which is likely stronger than either infection or vaccination alone. So you might not need this booster at all. But if you want to get this latest Omicron-specific booster (or if you are 65 and older), I would recommend at least 6 months from your last infection or booster before getting this Omicron-specific booster as I wrote in a piece in TIME today. Recent research studies during the Omicron variant era continue to demonstrate the benefit of an extended interval between doses in terms of increasing both neutralizing antibodies and memory B cells. A booster provides antibody protection for at least 6 months according to a recent study. Another study demonstrated that antibody levels stabilized 6 to 9 months post-vaccination for study participants both with and without previous infection. One of the aims of the Omicron-specific vaccines is to increase antibodies and prevent even mild infections. The antibody level plateau at the 6-month mark would thus signal an ideal time to boost with a BA.4/5-focused vaccine since a low pre-boost antibody level actually correlated with a greater fold increase post-boosting. To put it another way – high levels of circulating antibodies from short interval boosting may limit the added protection of another booster. Another recent study by the NIH showed the same concern after a recent infection, but even more drastically. Giving a booster 2 months after a recent infection actually abrogates effective B cell responses. Given that millions of people across the US were infected with Omicron (with recent study in JAMA showing that 56% of people who were infected with the Omicron variant were unaware of the infection), waiting at least 6 months since a recent infection is prudent.

Schaffner: There are several reasons that even fully vaccinated persons and those who recently tested positive should receive the updated vaccine. First, it is clear that by receiving the updated vaccine, the antibody levels in your bloodstream will increase. These increased antibody levels are thought to have a general correlation with the level of protection and its longer duration. Also, vaccination is associated with producing a broader spectrum of antibodies. Laboratory studies have indicated that this broader spectrum of antibodies is able to combat a wider range of COVID virus variants. Lastly, many persons have received their last booster several months ago, so there likely has been some waning of protection by now. Receiving the new, updated vaccine will restore levels of protection.

Gandhi: I doubt it except for children with immunocompromise. Children have robust immune responses to the first two vaccines and a low rate of severe disease, so are not likely to need Omicron-specific boosters

Schaffner: Studies of the new, updated COVID vaccine in children have been started. Frankly, a more immediate issue is that the majority of children have not yet received an initial COVID vaccination series and that is where the attention needs to be directed now.

Schaffner: There is a long, well-established history of the FDA approving and the Centers for Disease Control and Prevention (CDC) recommending certain vaccines without the benefit of prospective, controlled clinical trials. In each such instance, the new vaccines are updated versions of vaccines that have been used previously with success and are manufactured with similar methods. The most frequently cited example is the annual up-dating of the influenza vaccine. Another recent example is that in its October 2021 meeting, the CDC’s advisory committee voted to recommend the use of two new pneumococcal vaccines in adults. This decision was based on immunogenicity studies. There were no clinical trials of these vaccines. Additionally, over 600 million doses of the original COVID vaccine have been administered in the U.S. alone. On that basis, we have a very comprehensive assessment of their safety and effectiveness. Also, some 450 people each day continue to die of COVID. If we waited for the completion of clinical trials, the updated vaccine would not be available before the expected COVID surge this coming winter. Using the updated vaccine now gives us the opportunity to prevent many anticipated COVID hospitalizations and deaths.

Gandhi: Although there is absolutely biologic plausibility that the BA4/BA5 plus ancestral strain booster shots will increase neutralizing antibody data in humans, we only have mice data at this point (in <10 mice) that neutralizing antibody titers to BA5 are increased above the original booster shot. We also have clinical data from the BA1/bivalent vaccines in humans from which the CDC is extrapolating. Although we do use mice data for updating flu shots every year, this is the first time that the mRNA booster has been updated so getting human data with the BA4/BA5/ancestral strain bivalent vaccine may increase confidence in the vaccine. However, given that confusing messaging on boosting may have led to a lower booster uptake among older individuals in the US (with only 70% of our population over 65 having received the third shot) I would definitely encourage this booster most for those 65 years of age and older. Providing resources and attention to the group that needs an intervention the most due to being at the highest risk of a disease is good public health.