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An intubated patient inside a negative pressure room in the COVID-19 ICU at a hospital in Joplin, Mo., on Aug. 3.
An intubated patient inside a negative pressure room in the COVID-19 ICU at a hospital in Joplin, Mo., on Aug. 3. (Angus Mordant/Bloomberg)

Unvaccinated people in Los Angeles County were five times as likely to get infected with the coronavirus and 29 times as likely to be hospitalized as people who were fully immunized, newly released data from California show. It's the latest evidence that vaccines continue to reduce significantly the risk of severe illness — their fundamental purpose — despite the spread of the more contagious delta variant.

The report, published Tuesday by the Centers for Disease Control and Prevention, also demonstrates the limits of vaccines. They are not an impenetrable barrier to the virus. Some inoculated people are continuing to develop COVID-19, the disease caused by the virus.

Vaccine effectiveness — the statistical measure of protection from infection in the vaccinated population — has dropped as the delta variant has spread. On May 1, the report said, people who had not been immunized were more than eight times as likely to be infected as people who were fully vaccinated. That was before delta took hold, and on July 25, the ratio had dropped to about a fivefold greater risk.

But the vast majority of "breakthrough" cases among vaccinated people do not require hospitalization.

"Prior to delta, it did indeed appear that the vaccines were also very good at protecting against infection overall," Paul Simon, chief science officer for the Los Angeles County Department of Health, said. "But when delta emerged, there was a big change, because delta is so much more infectious. The vaccine didn't protect as well against infection."

The Los Angeles research joins a rising stack of studies that offer a mixed message about where the United States stands in its long war against the virus. Vaccinated people who had hoped they would be free from concern about infection have been sobered by evidence that breakthrough infections are more common than before the arrival of delta. At the same time, there has been little erosion in protection against severe illness and death.

"The vaccines are doing exactly what they promised us they'd do - they are keeping us from getting sick and dying, but with the delta variant, we are seeing more transmission than we saw with the alpha variant," Barbara Ferrer, director of the Los Angeles health department, said this month.

Still, some vaccinated people in Los Angeles have died. In the May 1 to July 25 span covered by the new report, 176 unvaccinated patients, 24 fully vaccinated patients and seven partially vaccinated patients died of COVID-19. Of the fully vaccinated patients, six were immunocompromised, and their median age — 78 — was higher than the median age of the unvaccinated people, which was 63.

The great majority of new coronavirus infections still occur in people who have not been immunized. But as vaccination rates increase, so will breakthrough cases as a percentage of new infections, experts point out.

"Simply, it's math. As we have more people vaccinated, more of our infections that we diagnose are going to be in vaccinated people," Oregon state epidemiologist Dean Sidelinger said. "It's not entirely unexpected."

Public health officials say recent research bolsters the CDC's view that even people who have had their shots would be wise to wear masks indoors under certain circumstances. Vaccinated people can spread the virus, the CDC has said. The viral load measured in people with breakthrough infections is comparable to the viral load in unvaccinated people, the Los Angeles study found.

New infections and hospitalizations have surged since mid-June, coinciding with the delta variant's rise to dominance, Simon said. The amount of virus in the community matters in the same way that the intensity of rainfall matters, he said.

"It's not a drizzle, it's a storm. Even if you're fully vaccinated, you should add that layer of extra protection — a raincoat, a mask — when you're out in the rain," Simon said. "Once we get this level of community transmission back down to a low level, I think people who are fully vaccinated will again have much more confidence. Not only are they protected against hospitalization and death, they're also very unlikely to get infected."

The Washington Post's Lena H. Sun contributed to this report.

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