The Food and Drug Administration granted full approval to the Pfizer-BioNTech coronavirus vaccine Monday, paving the way for more employers, schools and other organizations to mandate the shots.

“If you are not vaccinated, let this be the milestone that gets you there,” California’s public health officer, Dr. Tomás Aragón, said after the announcement.

Experts say that the highly contagious delta variant not only presents an unprecedented threat to those who are not vaccinated but has also provided ample evidence of the effectiveness of the vaccines.

In California, unvaccinated people are more than six times as likely to contract the coronavirus than those who have their shots, according to state data released Monday.

And in Los Angeles County, the state’s most populous, an unvaccinated person is as much as 25 times more likely to be hospitalized with the disease.

“That, in a sense, is our proof that vaccines work,” said Dr. Robert Kim-Farley, an infectious-disease expert at the University of California, Los Angeles’ Fielding School of Public Health.


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Similar to how states with low vaccination rates have been hard hit by the latest surge of the coronavirus, so have California counties where large swaths of the population remain unvaccinated.

Of the 20 California counties with the highest proportion of residents currently hospitalized with the coronavirus, 19 have a vaccination rate below the statewide average of 55%, according to The New York Times’ coronavirus tracker. (The percentages include children younger than 12 who are not yet eligible for the vaccines.)

Some of the most heavily affected counties in California are Yuba (36% vaccinated), San Bernardino and Butte (both 42%).

By contrast, the counties with some of the lowest hospitalization rates include Marin (75% vaccinated), Santa Clara (71%) and Santa Cruz (64%).

The proliferation of the delta variant has revealed a pattern that was less clear when the virus wasn’t circulating as widely, Kim-Farley told me.


“In a situation like this where we’re seeing basically COVID everywhere,” he said, it is easier to see the strong correlation between higher vaccination rates and lower rates of disease.

Still, additional factors may be at play, experts say.

A recent survey by the University of Southern California found that people who were unvaccinated were more likely than the vaccinated to go to a bar or a friend’s house and less likely to wear a mask or avoid large gatherings.

In other words, people who choose not to get vaccinated are also probably less worried about COVID-19 and take fewer safety precautions, contributing to their risk of falling ill, said Kevin Malotte, a professor emeritus of epidemiology at Long Beach State.

“Lack of mask measures, lack of worry about it, lack of vaccination are all kind of the syndrome, and I think that’s what we’re seeing correlate with the high rates,” Malotte said.

This article originally appeared in The New York Times.