Cases of people contracting Covid after being fully vaccinated are cropping up. Experts say it’s going to take a big effort to stem these infections.

Florida Rep. Vern Buchanan tested positive for Covid-19 after vaccination, according to a statement from his office Monday. Fully vaccinated entertainment journalist Catt Sadler warned her hundreds of thousands of Instagram followers not to “let your guard down,” after she got sick after caring for someone with Covid-19 who wasn’t vaccinated. Last Thursday, six players on the New York Yankees tested positive. This was the second instance of breakthrough cases on the team.

Breakthrough cases are also already cropping up in the Tokyo Summer Olympics. An alternate member of the US Olympics gymnastics team, Kara Eaker, who had been vaccinated tested positive for Covid-19 Sunday, her father confirmed to CNN affiliate KMBC Monday. So had basketball player Katie Lou Samuelson who confirmed on her Instagram account that she would not be able to compete in Tokyo.

For the Pfizer and Moderna vaccine full vaccination is after two doses. For the Johnson & Johnson vaccine it’s a single dose.

No vaccine is perfect

Covid-19 vaccines are highly protective against lab confirmed infection and seem to provide protection against the variants; however, a tiny fraction still become infected, just like they can with any other vaccine.

With other diseases like mumps or rubella, breakthrough infections are highly rare, Edwards said, because so many people have been vaccinated against those diseases, and mumps and rubella are in low circulation.

“The chance that a person who happened to be a nonresponder to the vaccine would come in contact with those diseases is very low,” said Edwards. “The reason why we are seeing more breakthrough infections with Covid is because there are so many unvaccinated people.”

Another example is the flu vaccine, which reduces the risk of getting sick between 40-60%, studies show. The Covid-19 vaccines are much more protective — as much as 95% effective at preventing severe illness, hospitalization and deaths.

Breakthrough infections by the numbers

We don’t know how many mild or asymtomatic breakthrough Covid-19 infections there are in the US. The CDC stopped counting in May.

It’s hard to draw any specific conclusions about the rate of infection from these numbers, but they are likely an undercount, according to the CDC. Surveillance data relies on voluntary reporting, and not all reporting is complete or even representative of total infections.

What scientists do know is that 99.5% of deaths from Covid-19 in the US right now are among people who are not vaccinated, US Surgeon General Dr. Vivek Murthy told CNN’s Dana Bash Sunday.

The CDC is monitoring breakthrough infections to identify which people are most likely to have breakthrough infections. The CDC said there are no unusual patterns so far.

Who may be more vulnerable to breakthrough infections

People with weakened immune systems are those who have had an organ transplant, are receiving chemotherapy for cancer, are on dialysis or are taking certain medicine that suppresses the immune system.

People who live in parts of the country with low vaccination rates may also have a greater likelihood of a breakthrough infection since they would be encountering more people with the disease.

What can you do to prevent a breakthrough infection

“If we want breakthrough cases to stop, then we need to have everybody else get vaccinated, so there’s no virus in circulation and then it won’t matter anymore,” Edwards said.

If more people are vaccinated, the coronavirus has fewer people it can infect. It also limits the number of new variants that can develop. More variants in circulation increase the likelihood that the coronavirus can evade the protection of the vaccines.

“If you are not vaccinated, you remain at risk,” said CDC Director Dr. Rochelle Walensky on Friday. “This is becoming a pandemic of the unvaccinated.”

When asked if people who are vaccinated should be doing anything different than they normally would on Sunday, Murthy told CNN that even with a breakthrough infection “which, again, happens in a very small minority of people — it’s likely to be a mild or asymptomatic infection.” He did say he would wear a mask indoors out of an abundance of caution if he is in an area with a large number of unvaccinated people.

“Again, even if the vaccine isn’t offering full protection it is offering a lot of protection,” Edwards said. “Even when they do not work as well in patients who are immunocompromised, they do provide some protection. That’s why it’s up to the rest of us to get vaccinated because we want to protect those people, the fragile and the elderly, and everyone else.”

“So please, I cannot say this enough,” Edwards said “Get vaccinated.”