SAN DIEGO, Calif. — Some doctors think there will be a need for a third COVID-19 vaccine "booster" shot.

"At some point, I think it's very likely," said UC San Diego Chief of Infectious Diseases Dr. Davey Smith. "When that point is, I don't know."

A lot is unknown about the need for a booster shot, as the vaccines have only been around for six months. That's barely given researchers time to determine how long they provide complete protection against COVID-19.

Dr. Smith says it's a process that can take years.

"We follow a whole bunch of people who got a vaccine, we measure their immune responses, and then we look and see how often people are getting infected over time," he explains.

"Once we start seeing more infections below a certain level (of antibodies), then we're like, okay, so if somebody has this level of antibody and it dips below that, then perhaps we need a booster at that point," said Dr. Smith.

But measuring antibodies may not be the only indicator of the need for a booster. As the pandemic continues, doctors discover new mutations and variants of COVID-19. There is a strong possibility one of those variants could be resistant to the vaccine, requiring a booster.

"Some of the mutations might be able to evade the immune protection," said Sharp Chula Vista Infectious Disease Specialist Dr. Hai Shao. "As a result, our neutralizing antibodies needs to be tweaked, to be made specifically against the new variants of viruses so that the level of protection can be modified and sustained."

Dr. Shao compares it to the flu, where the yearly vaccine includes several variants. He believes there may be a need for that type of vaccine for COVID-19.

"The likelihood is high," Dr. Shao said. "The question is when do we need the first booster, and how often do we need it after that."

In separate interviews last month, the heads of both Pfizer and Moderna said their vaccines would probably need booster shots. They did not specify when.

Current studies show the vaccines offer protection for at least six months and are effective against all known variants.

Research beyond that is ongoing.

In the meantime, Dr. Shao and Dr. Smith say everyone must get their initial doses of the vaccine to help achieve herd immunity.

"It is important for us to focus our attention and make sure that people who have not gotten the vaccine yet get it in time," said Dr. Shao.

"When you get a vaccine, you're protecting your community," said Dr. Smith. "I really want to get that message out."

This story was originally published by Jared Aarons at KGTV.