If you haven't gotten your Covid booster shot yet, public health experts say BA.5 is a prime reason to get it — and soon.

The Covid subvariant appears to be the virus's most transmissible strain thus far, powering a nationwide surge in new cases that hasn't slowed since March. That's a problem, even as hospitalizations and deaths remain relatively low: The longer Covid circulates, the more likely it is to mutate into a form that's both transmissible and severe.

Experts say booster shots are key to stopping BA.5 in its tracks, and all Americans age 5 and older are eligible five months after completing their primary vaccine series. But only 48.1% of eligible people in the U.S. have actually gotten boosted, according to Centers for Disease Control and Prevention data. More than 100 million lag behind, a problem for health officials already mulling the authorization of a second booster for most U.S. adults.

"The danger of not vaccinating, over the last couple of weeks, has significantly increased because the prevalence of the illness has gone up substantially," Kevin Dieckhaus, chief of the division of infectious diseases at UConn Health, tells CNBC Make It.

Here's how well booster shots protect against BA.5, what side effects to watch out for and how the omicron-specific booster shots likely coming this fall factor in.

Can boosters protect against BA.5?

The main job of a Covid vaccine is to prevent severe illness in case you get sick. It does help prevent infections in the first place, but it's not perfect in that regard — no vaccine is.

Booster shots essentially amplify those same benefits, says Ross Kedl, a professor of immunology and microbiology at the University of Colorado School of Medicine. He says booster-induced protection against severe illness holds up over time against every known Covid variant and subvariant, including BA.5.

The protection against infection wanes a bit after three or four months, Kedl says, but that's no reason to avoid getting the shot: Some help is better than no help, and there aren't any significant drawbacks for most Americans.

A second booster dose has also proven valuable for the Americans currently eligible. In adults over age 50, vaccine effectiveness against Covid-associated hospitalization dipped to 55% four months after a first booster shot, according to a recent study published by the CDC. A second booster restored that protection to 80% in just one week, the study noted.

Kedl says you should get a new booster the moment you become eligible for one. "The durability of your protection sustains even more each time you get a booster," he says. "Every time you get a booster, you get a little bit better protection against variants even as divergent as BA.5."

Will getting a booster now prevent you from getting an omicron-specific booster in the fall?

The short answer, according to a CDC statement last week: No.

"Getting vaccinated now will not prevent you from getting an authorized variant-specific vaccine in the fall or winter when they are recommended for you," the agency wrote. "Given recent increases in deaths and hospitalizations associated with the BA.5 variant, everyone should stay up to date with recommended Covid-19 vaccinations."

Dieckhaus says getting your first booster shot now could line up nicely with the anticipated timing of omicron-specific boosters in the fall by acting as a coverage bridge: Your temporary antibody boost against infection will likely decline right as those updated shots come out.

He notes that he used to advise people to consider waiting for omicron-specific vaccines, but that's no longer a safe option due to BA.5's transmissibility.

"If there's 'not a problem' or minimal problem, you can bide your time and wait. But unfortunately, now the virus is more active," Dieckhaus says.

Is there any downside to getting a booster?

After a booster shot, you can expect common symptoms like arm soreness at the injection site, muscle aches and fatigue, says Dieckhaus. Serious adverse effects are extremely uncommon, he adds.

Myocarditis, an inflammation of the heart muscle, sometimes occurs in young men after a Covid mRNA vaccine — but you're actually more likely to develop the condition by catching Covid than getting vaccinated against it, according to the CDC. Kedl says that if you didn't experience symptoms after your primary series, you probably won't from your booster, either.

The good far outweighs the bad, Kedl says. If you're worried about side effects — or simply experiencing vaccine fatigue — he recommends thinking about how the vaccines have already moved most Americans from panic mode to some level of normalcy.

"It's good to remember that it's really the vaccine side of things that have kept us able to operate normally in society," Kedl says, "It's made that safe, and continues to make that safe on an ongoing basis."

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