More than one in five Americans have been fully vaccinated against COVID-19. But vaccinated people can still get the coronavirus, though infections are exceedingly rare and typically quite mild.

Of the 75 million people across the US who were fully vaccinated as of Tuesday, some 5,800 had still tested positive for COVID-19, in what's known as a "breakthrough infection," according to new data from the Centers for Disease Control and Prevention (CDC).

A few of those infections in vaccinated people were severe. About 400 of the 5,800 people with breakthrough infections, or 7 percent, required hospitalization, and 74, or 1.3 percent, died from the disease, the CDC said in a statement.

The report is another reminder that, as Dr. Anthony Fauci and other public-health experts have been stressing for months, vaccination is not a one-and-done way to end the pandemic. Wearing masks and social distancing remain critically important until we have enough immunity to stop the virus' circulation.

'The vaccine is working as expected'

These breakthrough infections are to be expected and are in line with how other vaccines work, the CDC said.

"We expect thousands of vaccine breakthrough cases will occur even though the vaccine is working as expected," the statement said.

"With the number of breakthrough cases, I think the important thing is to look at what the denominator of vaccinated people is," Fauci, the top infectious-disease expert in the US, said during a White House press briefing last week.

Breakthrough infections have occurred in about 0.008 percent of the people who've had their COVID-19 shot(s) and allowed them two full weeks to take effect. Fatal breakthrough cases have been very rare; the Minnesota Health Department tallied zero deaths in the first 89 breakthrough infection cases in the state.

"It's important to know that even if someone is vaccinated and then goes on to be one of the few unfortunate people to develop a breakthrough case, there still can be some level of protection provided by the vaccine," Kris Ehresmann, the department's infectious-disease director, said during a briefing in March.

The breakthrough rate of infections in Minnesota at that time was about 0.01 percent.

The new figures from the CDC also suggest that the authorized COVID-19 vaccines are nearly perfect at preventing death, as clinical trials had suggested: With 74 fatalities in 75 million people fully vaccinated, the vaccines appeared to be 99.99 percent effective.

Some people who are vaccinated and subsequently get infected may not even know it either, as about a third of the breakthrough infections (29 percent) were asymptomatic, discoverable only with a test, the CDC said.

40 percent of the breakthrough infections were in fully vaccinated people over 60

The risk of a breakthrough infection might not be equal among everyone who's been vaccinated. The CDC said about 40 percent of the breakthrough infections reported had been discovered in people over 60, a demographic in which vaccines are often less effective.

"It is likely that elderly individuals, particularly if they are frail and have underlying conditions, might not have responded as well to the vaccine," Fauci said.

"When someone is already elderly and may or may not have an underlying condition, that it is unfortunate but not surprising that you might have a couple of deaths."

The CDC said it created a nationwide "vaccine breakthrough database" for state health departments to help the federal government monitor cases.

"People who have been fully vaccinated should keep taking precautions in public places like wearing a mask, staying at least 1.83 metres apart from others, avoiding crowds and poorly ventilated spaces, and washing their hands often," the CDC said.

This article was originally published by Business Insider.

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