(NEXSTAR) – Americans over 50 and those with underlying health conditions are now eligible for a second booster shot against COVID-19 – the fourth in the vaccine series for many. Populations eligible for the second booster are able to get it as soon as four months after their last shot – but is there any reason to hold off on the extra dose?

Evidence we’ve gathered so far indicates the protection you get from a COVID-19 vaccine wanes somewhat over time. It takes two weeks for your immune system to fully react to a dose, but timing your dose perfectly right is tricky.

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“If we could predict exactly when the surge is going to hit us, then absolutely you’d want to time that to get your vaccination two weeks before those cases,” said Dr. Brian Garibaldi, director of Johns Hopkins’ biocontainment unit. “But I think it’s unclear when that might be, and there’s reason to believe that may be happening soon.”

Dr. Bob Wachter, chair of the department of medicine at the University of California, San Francisco, agreed on Twitter. He said trying to time your second booster for right before the surge “reminds me of trying to time the stock market,” which he quipped “rarely works.”

“I’d get the shot,” Wachter tweeted.

As we see other countries experiencing spikes driven by the BA.2 variant of omicron, epidemiologists expect the U.S. will see more COVID-19 cases soon.

“It’s very likely we are going to see an increase in cases sometime in the next few weeks to a month here in the U.S.,” said Garibaldi. Though he said he doesn’t expect the surge to be nearly as big as the winter spike in COVID-19 cases caused by the BA.1 variant of omicron.

It’s not clear yet how much additional protection a fourth shot offers. An early study out of Israel indicates a fourth shot greatly reduced the risk of dying from COVID-19 for older people, but the study hasn’t been peer-reviewed and there hasn’t been enough time yet to collect additional evidence in the U.S.

However, the risks associated with additional booster shots are very small.

“These shots are safe and provide substantial benefit,” the Centers for Disease Control and Prevention wrote in its most recent announcement of booster eligibility. “During the recent Omicron surge, those who were boosted were 21-times less likely to die from COVID-19 compared to those who were unvaccinated, and 7-times less likely to be hospitalized.”

While everyone over the age of 12 is eligible for a booster shot, only 30% have received one, according to CDC data. Garibaldi urges people eligible for their fourth shot to do so, but even more importantly, he urges people to get their third shot if they haven’t yet.

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“Now’s the time to do it to really give you the best chance of having protection from severe disease or death if there is another surge or another variant that comes along,” he said.