‘The virus doesn’t care — day or night’: Is there real science behind COVID curfews?

Gov. Gavin Newsom on Thursday announced a curfew for California counties in the purple reopening tier to control the spread of the coronavirus. But experts are mixed on whether such measures have any tangible effect.

Much of California has been pushed back into that most restrictive tier to control the recent virus surge. Those counties, including six in the Bay Area, will therefore be subject to the 10 p.m.-5 a.m. “limited Stay at Home Order” which, according to a tweet by Newsom, will last one month.

Los Angeles County, where case counts have doubled in the last week, had already announced a curfew starting Friday mandating all restaurants, bars, wineries and non-essential businesses to close from 10 p.m. to 6 a.m.

The measure is similar to one New York imposed statewide last week, and what other European countries and cities have experimented with to control new waves of the virus. Newsom said this week he and officials were still combing through the research, but experts say the science behind curfews during a pandemic is still relatively scant.

Health officers across the Bay Area have repeatedly said that indoor gatherings, particularly those involving multiple households, are the main drivers of the recent local surge. But officials say enforcing rules against those types of gatherings is virtually impossible. Many of them also occur during the day, when the weather is usually more amenable to gathering.

“The virus doesn’t care — day or night,” said Mark Cullen, an infectious disease expert and former professor at Stanford University. “It’s really just a way of getting at behavior modification, but it just seems like an odd one that doesn’t in and of itself address the problem.”

Cullen said he’s skeptical that a curfew measure would do much to slow the spread of the coronavirus in the Bay Area, or anywhere else. It’s not clear whether more social activity occurs at night, he said, adding that regulations should be directed toward actual behavior (no indoor dining, or no bars) rather than the indirect emphasis on a time frame.

There’s also the worry that such curfews could drive more people indoors, rather than gathering in more socially distanced outdoor settings, said infectious disease professor Lee Riley. He said he was particularly worried about potential impacts on the younger and college-age population in the Bay Area.

“It seems very cosmetic, and not really getting at the fundamental issue of where transmissions are occuring,” he said. “My feeling is that I don’t think it’s going to have a big impact.”

Major studies are lacking that show any significant effect. They also face a scientific challenge: There is no viable control group.

One French study — which has not been peer-reviewed — analyzed the spread of the coronavirus under curfews that began Oct. 30. The study noted that the spread “decreased notably with curfew measures and even more so for the vulnerable population group, that is, for people under 60.” The curfew did not impact the spread within younger population groups, the study found.

Another study by the London School for Hygiene and Tropical Medicine, which looked at two measures — only six contacts per person, and a 10 p.m. curfew in restaurants and pubs — found no effect for each one. Of the thousands of people who were surveyed, 42% had the same amount of contacts before and after the measure was introduced, and 26% saw more.

Twenty-four percent of people said their contacts had increased after the 10 p.m. curfew, while 50% said their number of contacts had stayed the same.

Annie Vainshtein is a San Francisco Chronicle staff writer. Email: avainshtein@sfchronicle.com Twitter: @annievain

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