Here’s what to know about breakthrough infections and the Delta variant.



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Amid new evidence that vaccinated people with breakthrough infections can carry as much coronavirus as unvaccinated people, the Centers for Disease Control and Prevention has urged people in high-transmission areas to wear masks in public indoor spaces, regardless of their vaccination status.

Vaccines remain highly effective at preventing severe illness and death, but the highly contagious Delta variant and persistent vaccine refusal have taken the United States in an unexpected direction. Reported cases have spiked to the highest levels in six months.

Still, most vaccinated people with breakthrough infections are likely to have mild symptoms, and each exposure to the virus is an opportunity for the immune system to strengthen its defenses against future variants.

Here’s what’s useful to know about breakthrough infections and the Delta variant:

The vaccines were intended to prevent hospitalization and death, in large part the result of damage to the lungs and other organs. The vaccines produce antibodies in the blood that prevent the coronavirus from taking root in those organs.

Early in the infection, when people are most likely to be contagious, the Delta variant seems to replicate in amounts that are perhaps 1,000 times as much as those seen in people infected with other variants, defeating immune defenses in the nose and throat.

Some experts believe breakthrough infections are likelier after exposure to the Delta variant than to prior forms of the virus. Even when more recent data becomes available, however, it still is likely to show that most hospitalizations and deaths from Covid-19 occur in unvaccinated people.

In some rare cases, breakthrough infections may lead to persistent symptoms. “Long Covid” is a poorly understood set of symptoms that can affect people for several months after an active infection has ended. A couple of small studies have investigated how common or severe long Covid may be after breakthrough infections. It is likely to be rare, some experts say, because breakthrough infections are uncommon and shorter in duration.

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