More than 4,100 people in the United States have been hospitalized or died from Covid-19 despite being fully vaccinated, according to new data from the Centers for Disease Control and Prevention.

So far, at least 750 fully vaccinated people have died after contracting Covid, but the CDC found that 142 of those deaths were asymptomatic or unrelated to Covid-19, according to Monday’s data released on Friday.

3,907 people were hospitalized with groundbreaking Covid infections despite being fully vaccinated. Of these, more than 1,000 of these patients were asymptomatic or their hospital admissions were unrelated to Covid-19, the CDC said.

“Was to be expected,” said Dr. Paul Offit, a top advisor to the Food and Drug Administration on childhood vaccines, told CNBC. “The vaccines are not 100% effective, even against serious illnesses. Very small percentage of the 600,000 deaths.”

Breakthrough cases are Covid-19 infections that bypass vaccination protection. They are very rare and many are asymptomatic. The vaccines are highly effective, but they don’t block every infection. Pfizer and Moderna’s Phase III clinical trials found their two-dose regimens were 95% and 94% effective at blocking Covid-19, respectively, while Johnson & Johnson’s one-time vaccine was 66% effective in its trials % was effective. However, all three have been shown to be extremely effective in preventing people from getting seriously ill with Covid.

The CDC doesn’t count every breakthrough case. Mai stopped counting all groundbreaking cases and now only counts those that result in hospitalization or death, a move the agency has been criticized for by health experts.

Most Americans have received at least one injection of the two currently approved mRNA vaccines. The US has given 178.3 million vaccinations and fully vaccinated 46% of the population.

“You are just as likely to be killed by a meteorite as after a vaccine on Covid,” said Dr. Peter Chin-Hong, an infectious disease expert at the University of California San Francisco, told CNBC. “By and large, the vaccines are tremendously powerful.”

Efficacy rates for variants like Alpha and Delta decrease slightly, with studies showing 88% effectiveness against the Delta variant after two doses of the Pfizer vaccine. It was unclear whether any of the breakthrough cases reported were caused by variants.

In Israel and the UK, concerns about the Delta variant are mounting on growing reports of breakthrough infections.

Even with 80% of adults vaccinated, Chezy Levy, director general of Israel’s Ministry of Health, said the Delta variant is responsible for 70% of new infections in the country. Levy also said a third of these new infections occurred in people who were vaccinated.

In the UK, Public Health England published a report that found that from June 8-14, 26 out of 73 deaths from the Delta variant occurred in fully vaccinated people. Most of the deaths occurred in unvaccinated people.

“Determining whether hospital admissions and deaths are more prevalent in immunocompromised patients, and the type of vaccine received will be important for future guidance,” said Chin-Hong.

On June 7, the CDC received reports of 3,459 breakthrough cases resulting in hospitalization or death. On June 18, that number was updated to 3,729, an increase of 270 cases. Today the number stands at 4,115.

The overwhelming majority, 76%, of hospital admissions and deaths from breakthrough cases occurred in people over the age of 65.

“We don’t have the years of data we have for vaccines against other airborne pathogens – so it’s really important that the CDC provide updated reports on breakthrough cases,” said David Edwards, aerosol scientist and professor at Harvard University opposite CNBC.

The CDC says their numbers are “likely to be an undercount” of all Covid infections in vaccinated people as the data is based on passive and voluntary reporting.

CNBC’s Berkeley Lovelace Jr. contributed to this report