New York mobilized its National Guard for a two-week lockdown of a city suburb with a coronavirus cluster. Washington state is pondering mandatory public restrictions to limit the illness’s spread. Santa Clara County has banned gatherings of more than 1,000 people.

But California isn’t yet taking steps like New York and Washington. Other Bay Area counties haven’t gone as far as Santa Clara County in banning public gatherings. And while a few schools have closed or stopped in-person instruction, others remain open.

As the potentially deadly coronavirus disease known as COVID-19 spreads in the U.S., the response of public agencies has seemed to be inconsistent and even chaotic. But public officials and health experts say there are reasons for the seemingly scattered approach.

Catherine Troisi, an infectious disease epidemiologist at The University of Texas Health Science Center at Houston, said Tuesday that “social distancing” measures such as closing schools, games and other gatherings has worked in the past. But those measures come with costs of their own. And with a new virus that behaves differently — and in ways experts don’t yet fully understand — there’s not much agreement on how much “social distancing” is enough.

“It depends where you are in your comfort with risk,” Troisi said. “If we wanted to make sure there was no spread at all, we could tell everybody that for two weeks you can’t be in contact with another person. That’s a little overboard. On the other end is to do nothing. So in between is a wide range of possibilities.”

Though California has almost as many confirmed coronavirus cases as Washington and New York, Gov. Gavin Newsom said Tuesday that he’s not considering steps as extreme as those taken by governors in those states.

“This is a dynamic process,” Newsom told reporters, “but I can assure you we’re not there yet. We have second-to-none the best guardsmen in United States, but right now we have not identified a need to use them.”

Newsom also said that while California is advising local officials in consultation with federal health authorities, it is deferring to their judgment as to what steps should be taken to keep the virus contained.

“Each and every district, each and every county, conditions are different,” Newsom said. “What we’re doing is a bottom-up process, not top-down process. We are providing guidance and sending that down, but also trying to standardize guidance.”

In Santa Clara County, there have now been 43 cases — 21 of which were not associated with foreign travel or exposure to those already sick — and one fatality, county health officer Sara Cody told the board of supervisors Tuesday morning. The county reported two additional cases in the afternoon.

“That indicates a significant level of community transmission,” Cody said. “This is all hands on deck.”

By contrast, Solano County has recorded six cases involving residents, including some with no known association to foreign travel or others who were sick. That number doesn’t include nonresidents quarantined at Travis Air Force Base after being returned to the U.S. from overseas or from cruise ships where infections surfaced.

But Solano County — which has recorded no fatalities — has not gone as far as banning large public gatherings, only advising that events aimed at older adults, who are more susceptible to the disease, be postponed. Solano County Public Health Administrator Jayleen Richards said Tuesday that those decisions are made after consulting with federal and state health authorities.

“We’re continuing to look at the data and how COVID-19 is affecting the population, and that’s how we’ll make decisions,” Richards said.

Newsom had no problem with that.

“The situation is distinct in Santa Clara now,” he said. “Santa Clara, I thought wisely, advised social distancing for crowds over 1,000. I think it’s likely others will do the same.”

Public health officials said part of the reason for the variations in policies is the rapidly evolving knowledge about the virus and how it behaves.

A month ago, efforts were focused on screening people with fevers, coughs and sneezes who had traveled to “hot zones” in China — where the outbreak began in December — and other nearby countries, informed by the belief the virus chiefly spread by people who were sick and exhibiting symptoms.

Since then, however, evidence has emerged suggesting the illness can be carried and spread by people with mild or no symptoms. The virus incubates anywhere from two to 14 days or more, and those infected spread it to an average of two or three others, Cody said. Moreover, limited availability of tests has made it difficult for health officials to get a picture of how many may be carrying and spreading the disease in their communities.

“We don’t understand how much those people may be contributing to accelerating the spread,” she said.

The World Health Organization on Tuesday advised “that measures that restrict the movement of people during this outbreak should be proportionate to the public health risk, short in duration and reviewed regularly as more information about the virus … becomes available.”

Globally, the virus has infected at least 113,000 people and killed some 4,000, the vast majority in China. More than 32,000 cases have been reported in more than 100 other countries, with more than 800 deaths. In the U.S., there have been at least 900 cases and more than two dozen deaths.

California now has 157 confirmed cases, not including passengers from the Grand Princess cruise ship currently docked in Oakland, where 19 crew members and two passengers tested positive. There have been two deaths in the state, including a man who traveled on the Grand Princess in February.

Troisi said the social distancing approach was developed in response to influenza pandemics such as the “Spanish flu” in 1918. During that epidemic, in St. Louis, early closures of schools, churches, dance halls and theaters and bans on public events slowed the spread and saved lives, she said. By contrast, Philadelphia delayed such steps and allowed the Liberty Loan Parade to proceed that September, Troisi said — a decision that was followed by a spike in death rates the next month.

But it is not as clear how quickly local authorities should act with COVID-19, Troisi said, because the new virus isn’t the same as the flu. Influenza affects children more than COVID-19 does and spreads quickly among them, so aggressive school closures made sense.

With COVID-19, local officials are concerned that the cost of closing schools might outweigh the benefits to stopping the spread of a disease that does not appear to be deadly in children. Moreover, closing schools can force parents — including doctors and nurses — to stay home as well.

Cody said she has been in touch with officials in the Seattle area, which last week instituted the farthest-reaching school closures, and said Tuesday that keeping local schools open is “one of the toughest decisions we have.”

“What I can tell you right now is that I think the harms of widespread school closure outweigh the benefits,” Cody told the board.

California health authorities last weekend issued new guidelines to local officials in weighing closures of schools and cancellations of public events. For schools, the measures ramp up for counties with two or more cases of COVID-19 spread in the community, more so if one student, teacher or staff member falls ill. In cases with multiple students or teachers sickened, the state advises that closures could be considered in consultation with local health authorities.

For counties with coronavirus spreading in the community, the state advises that local officials consider canceling or postponing nonessential events.

Troisi said it’s not unreasonable to have such different responses in different places.

“The situation is different in different parts of the country,” Trioisi said. “It’s been since at least 1957, maybe 1918, since we’ve had a pandemic like this. We don’t know all the answers. We’re trying the best we can, but there are legitimate differences of opinion on what should be done.”