To help clarify the influx of information and evidence about the coronavirus as the pandemic evolves, the New York Times' Apoorva Mandavilli breaks down the most recent information on breakthrough infections and the delta variant in five key takeaways.

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1. Vaccines aim to prevent severe illness—not infection.

According to Mandavilli, Covid-19 vaccines were designed to prevent the worst outcomes of infection, primarily hospitalization and death due to severe organ damage.

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To do so, she writes, vaccination produces antibodies in the blood that stop the coronavirus from entering the lungs and other organs—not the nose and throat.

Some vaccine-produced antibodies have also been found in nasal secretions and saliva, but those are likely in sufficient amounts to thwart early variants of the coronavirus. However, they may be unable to defend the nose and throat against the delta variant specifically, which replicates potentially 1,000 as much as other variants, she writes.

"[Delta is] just way more virus that's going to potentially overwhelm that initial firewall," said Jennifer Gommerman, an immunologist at the University of Toronto.

Gommerman added that vaccines would have to produce long-lasting antibodies in both the blood and nose to prevent infections as well as severe illness from the delta variant, which is a "really tall ask for a vaccine."

2. Breakthrough infections are likely more common than known—but probably don't result in severe illness.

Most current estimates of breakthrough infections use data from before the delta variant's dominance in the United States, so it's not clear exactly how common breakthrough infections are now, Mandavilli reports. In addition, there is not enough testing being done to get an accurate count.

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Mandavilli adds that while some infectious disease experts believe breakthrough infections are more likely to happen after being exposed to the delta variant than other variants, most Covid-19 hospitalizations and deaths are still likely going to occur, primarily among the unvaccinated.

"Epidemiologically and clinically, I've still not seen really any severe cases among fully vaccinated people who are immunocompetent," Abraar Karan, an infectious disease fellow at Stanford University, said. "The pattern that I'm seeing is still primarily unvaccinated who are making it to the ICU."

According to CDC, as of Aug. 2, more than 7,500 people with breakthrough infections had been hospitalized or died. Additionally, a New York Times analysis of 40 states found that fully vaccinated people make up less than 5% of Covid-19 hospitalizations and less than 6% of Covid-19 deaths.

3. Breakthrough infections are likely more dangerous for the unvaccinated.

Ultimately, according to Mandavilli, experts believe breakthrough infections are also more likely to put people who are unvaccinated—or people who are immunocompromised—at risk than people who are vaccinated.

She cites CDC data showing that while vaccinated people are less likely than unvaccinated people to get infected, in the rare instances when vaccinated people do get infected, they appear to carry as much virus in their noses and throats as those who are unvaccinated. This means they can transmit the virus to other people very soon after they are infected, even before they develop symptoms.

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As a result, Mandavilli writes, breakthrough infections could then add to viral transmission within a community. This could further the spread of the coronavirus and put unvaccinated people at risk—although any transmission will likely occur less often and last a shorter time.

4. Breakthrough infections, in rare cases, lead to long Covid-19.

According to Mandavilli, most fully vaccinated people who become infected will probably experience only mild symptoms. However, in some cases, breakthrough infections may lead to lingering symptoms, or "long Covid," that persists for several months after an initial infection.

So far, only a few small studies have looked at the prevalence and severity of long Covid after breakthrough infections, she writes. For instance, a study in Israel found that 7 out of 36 people with breakthrough infections experienced symptoms lasting more than six weeks. And a survey of Covid-19 survivors found that 24 out of 44 people with symptomatic breakthrough infections reported lingering symptoms.

"We really need a wider national or even international survey," Akiko Iwasaki, an immunologist at Yale University, said. However, according to Mandavilli, the overall rate of such cases is likely low, given breakthrough infections are themselves uncommon and typically brief.

5. Breakthrough infections may provide an 'advantage.'

According to Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health, natural infections among those who are already vaccinated may act like booster shots and boost the immunity offered by vaccination, in part by educating the immune system on how to recognize and address a new variant or version of the original virus.

Over time, he said, whether through booster shots or new infections, people's immune systems will learn enough about the virus to suppress new variants—although he added that "we're not there yet." (Mandavilli, New York Times, 8/14)