With about half of adults in the U.S. having received at least one dose of the COVID-19 vaccine and about a third of the adult population fully vaccinated, many are wondering whether additional shots will be necessary in the future.

Studies have shown that both the Pfizer and Moderna vaccines remain effective six months after they’re administered. Both companies have also said that a third booster shot will likely be needed, and Moderna plans to have its new dose available by the fall.

Local experts emphasize that the longevity of the current coronavirus vaccines as well as how often additional shots may be necessary are topics that are currently being researched.

Eugene Curley, Medical Director of Infection Control for WellSpan Health, says in an email, “We still do not know how long immunity will last from these vaccines. Participants from the phase 3 clinical trials will be followed for 2 years to help determine the duration of immunity.”

Joseph Kontra, Chief of Infectious Diseases at Penn Medicine Lancaster General Health, notes in an email that researchers are certain that the Pfizer and Moderna vaccines are effective for six months, and some scientists speculate that protection from the vaccines could last more than a year. However, at the moment, “we just don’t know.”

Factors that could influence immunity duration and the need for additional shots include individual immune response, virus variants and delayed herd immunity, writes Curley.

At the beginning of April, Anthony Fauci, President Biden’s chief medical adviser, told Nexstar station WPIX that protection from the COVID-19 vaccines “almost certainly…is going to diminish over time.” But how quickly it will diminish is still unknown as the vaccines have only been studied for six to eight months.

“We may need [a booster shot]…but we don’t know exactly when because we don’t know when it gets to the point when it gets below the level of protection,” Fauci said.

Richard Zimmerman, professor of family medicine and public health at the University of Pittsburgh and a former member of the U.S. Centers for Disease Control and Prevention’s Advisory Committee for Immunization Practices, shared via email, “I speculate that COVID vaccine boosters will eventually be needed for both intrinsic immunity and escape variant reasons.”

Kontra also agrees that COVID-19 variants may necessitate a modified vaccine booster. In an earlier interview, he told abc27 that all three vaccines currently deployed in the U.S. are effective against the U.K. variant, which is the dominant variant in the U.S., but they are less effective against the Brazilian and South African strains.

Distribution of the Pfizer and Moderna COVID-19 vaccines got off to a slow start in Pennsylvania, with many struggling to find available appointments both in the state and around the country. In February, abc27 spoke with Kontra about whether the distribution of future COVID-19 shots might go more smoothly.

Comparing the coronavirus vaccines to the common annual flu shots, Kontra noted that “companies know to ramp up influenza [vaccine] production going into the flu season,” and they study common strains of influenza in other parts of the world to determine which strains to include in the U.S. flu vaccine.

“That’s a very well worked out process, there are many manufacturers of influenza vaccine, and that kind of production line could in fact be what happens with COVID-19,” Kontra said. If additional COVID-19 vaccines are needed, they may be developed using a similar process. “The strains that are circulating will be analyzed, the vaccine will be adjusted, and when the time comes to vaccinate everyone, we’ll get up and running,” he hypothesized.