Q&A: Why are a small number of vaccinated people catching COVID-19?

The infection of two vaccinated Miami-Dade County Commission officials working at the collapsed Surfside condo tower has again thrust so-called “breakthrough” coronavirus cases into the spotlight.

Miami-Dade County Commission Chairman Jose “Pepe” Diaz and his chief of staff, Isidoro Lopez, both of whom are vaccinated, came down with flu-like symptoms Sunday and later tested positive for the virus, according to a statement reported in the Miami Herald.

Health experts are concerned that breakthrough cases, while infrequent, may erode confidence in the effectiveness of the vaccines, making it tougher for the nation to achieve herd immunity.

Here’s a look at breakthrough cases and what they mean for you.

How does a breakthrough case happen?

It’s a common misconception that vaccines prevent people from catching infections.

Instead, vaccines give your immune system “a leg up in recognizing (a virus) as foreign when it enters your nose or your lungs,” said Cindy Prins, an epidemiologist at the University of Florida. “Getting the vaccine is like giving your immune system a most-wanted poster. You’re telling your immune system: ‘Keep an eye out for this, and if you see it, attack it very quickly.’”

But this early warning system doesn’t work every time. In a small percentage of cases, the immune system can’t act fast enough, and the virus spreads enough to cause an infection.

No vaccine is 100 percent effective, but the two-shot Pfizer and Moderna coronavirus vaccines are rated as about 95 percent effective. By comparison, the 2020 influenza vaccine was 50 percent effective against influenza B/Victoria viruses and 37 percent effective against influenza A.

How often do breakthrough cases happen?

As of July 6, the U.S. Centers for Disease Control and Prevention had recorded 5,186 cases of breakthrough infections where the patient was hospitalized or died. That’s out of more than 157 million people who were fully vaccinated at that time.

The number of cases is almost certainly higher, because the vaccine greatly reduces the chance of severe infections, said Michael Teng, an immunologist at the University of South Florida.

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“If you had an asymptomatic case, you’d never know it unless you happen to get tested on the right day,” Teng said.

Vaccines remain incredibly effective at preventing severe illness and death due to COVID-19, health experts said.

“Preliminary data from several states over the last few months suggest that 99.5 percent of deaths from COVID-19 in the United States were in unvaccinated people,” CDC director Dr. Rochelle Walensky said in a June 8 press briefing.

A separate analysis from the Associated Press indicates that about 0.8 percent of deaths in recent months were in vaccinated people.

What about the delta variant?

Data released last week from the CDC indicates that the delta variant accounted for 13.2 percent of new infections in Florida between May 22 and June 19.

What concerns Dr. Kartik Cherabuddi, a professor of infectious diseases at the University of Florida, is how infectious the variant is compared to previous strains and how quickly it replicates once in the body.

Researchers at the Guangdong Provincial Center for Disease Control and Prevention in China found that the delta variant grows about 1,000 times faster in the respiratory tracts of infected individuals, compared to the original strain of the virus and spreads about 225 percent faster among unvaccinated individuals.

And as the variant spreads among unvaccinated people, it increases exposure and could lead to more breakthrough infections, Cherabuddi said.

There is no state or federal data available about how many breakthrough cases are linked to the delta variant.

I’m already vaccinated. What else can I do to protect myself?

Even vaccinated people might want to take additional precautions, said Teng, such as wearing a mask or opening a window when in enclosed spaces with a lot of people.

The CDC does not suggest that vaccinated individuals wear masks indoors, but the World Health Organization continues to recommend facial coverings as a preventative measure for everyone.

Vaccinated people should remember that they can still contribute to community spread of the virus, said Jason Salemi, an epidemiologist with the University of South Florida.

“Being a fully vaccinated person, I know the odds that I’m going to get the virus and get severely ill are very, very, very low,” he said. “But when I go in a public setting and I’m around other people, I choose to continue to wear a facial covering and socially distance from people.”

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