PITTSBURGH (KDKA) – Knowing the majority of COVID-19 hospitalizations are people who are unvaccinated is a small consolation when the vaccinated hear stories of breakthrough cases.

By definition, breakthrough cases are when the coronavirus has broken through the wall of protective vaccinations and gotten someone sick.

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Breakthrough cases range from minor discomfort to some rare instances of death.

CBS News medical expert Dr. David Agus says the breakthrough cases are disconcerting to the vaccinated while at the same time mildly reassuring.

“I mean breakthrough is real, and so these vaccines are pretty remarkable in that they have really built powerful lower respiratory immunity to protect the lungs really well,” he says.

He adds it’s the lung infections that lead to hospitalizations, intensive care, and death.

“Very few are hospitalized and very few have a serious infection,” Dr. Agus says about the breakthrough cases. “The ones that do, it’s almost in all cases they have other medical conditions, ongoing, it’s not directly from the virus.”

However, it’s the deaths that get the headlines.

“No question about it and they are extremely, extremely rare,” he says.

“We do know that when people do get infected. After being vaccinated, they don’t have a tendency to be as sick as people who never got vaccinated and they get much more sick,” Cleveland Clinic’s infectious disease specialist Dr. Esper Delta adds.

When it comes to vaccine efficacy the vaccines are exceeding their clinical trials, but Dr. Delta adds, “Nothing in life is ever 100%.”

This is why both experts say no one can let up on the precautions drilled into us over the last two years of washing hands, wearing a mask, and getting vaccinated.

While the Delta variant is the predominant strain of the virus nationwide, Doctors are also dealing with the Mu variant now.

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Dr. Agus says the Mu is not new.

“It is now seen in 49 of the 50 states in the United States,” he explains. “It peaked about a month and a half ago, and the numbers are coming down some, but our sequencing data is delayed.”

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He says so far the vaccines have stood up to the Mu challenge.

Dr. Agus, who is the founder and CEO of the Ellison Institute of Transformative Medicine at the University of Southern California, says Mu has not taken off as Delta has.

“I think we’re keeping a very close eye on it,” he says. “We’re not tremendously worried at the present time because the numbers are still low and they have not gone up over the last several weeks while it’s been here, but still it’s something that we have probably paying close attention to and we’re following on a daily basis.”

Dr. Agus is optimistic about the next month in the battle with all the COVID variants.

“What we’re going to see is that over the next several weeks, significant fall in the number of cases in the United States, as people get immunity from getting delta, and the 53% of the country that’s been double vaxxed that will build enough immunity I think to have a fall-off in the virus,” he explains.

And Dr. Agus says the boosters will help.

“No question about it booster shots raised immunity four to tenfold,” Dr. Agus says. “If you look in countries like Israel there’s a dramatic drop in symptomatic breakthrough infections and people who have received a booster shot.”

The first boosters will be available in a few weeks and Dr. Agus says it takes about a week after injection for the booster to become effective.

Although he says it can’t come soon enough as hospitals, especially in unvaccinated and unmasked areas, are over capacity and having to turn people away.

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Something Dr. Agus says should never happen in this country.