A new study estimates that as many as 270,000 Californians are infected with the coronavirus, more than 10 times the number reported, and that New York is much worse off than even the most pessimistic disease trackers believe.

The 61-page report appears to bolster the long-held belief by medical specialists that there is a huge population of untested virus carriers who could be infecting others, but Bay Area epidemiologists are skeptical that the infection rate could be as high as the authors predict.

Dayton Thorpe, an independent San Francisco data scientist, and Kelsey Lyberger, a doctoral candidate in population biology at UC Davis, used extrapolations from reported deaths in the United States to postulate that the number of people infected with the coronavirus in all 50 states is 39 times higher than reported.

The study claims almost half of New Yorkers are probably infected, by far the most in the nation.

“There are more cases than I thought,” Thorpe said. “That is absolutely a concern. The virus is in more people and in more places” than anyone thought.

Local experts, however, questioned the report, particularly the projected numbers in New York. They say that even if the model is sound, coronavirus projections like this one are based on incomplete and faulty data, particularly in the United States, which is hampered by a lack of testing.

“Sounds like a massive stretch to me,” said George Rutherford, a professor of epidemiology and biostatistics at the UCSF School of Medicine. “I’m highly, highly skeptical.”

But the projections aren’t completely out of line with previous studies, including one in the academic journal Science last month that estimated 86% of all infections were undocumented. Harvard epidemiologist Marc Lipsitch told the Atlantic in February that 40% to 70% of people worldwide could be infected within the year.

Gov. Gavin Newsom warned last month that as many as half of Californians — about 20 million people — could end up with the coronavirus in two months if nothing were done to stop the spread. To prevent that, the entire state has been shuttered since then. Experts now say Newsom’s projection would have taken a year or two even if no actions were taken to halt the spread.

The general consensus among the various studies seems to be that there are about 10 infections for every 1 that is diagnosed.

Thorpe and Lyberger said they used the same method that researchers at Imperial College London used to calculate infection rates around the world, including the prediction that 2 million people could die if the United States did nothing.

It was the Imperial College study that purportedly persuaded President Trump to release a 15-day plan on March 16 asking Americans to work from home and take other measures to slow the spread of the virus.

The latest study, which was provided to The Chronicle but has not yet been published, took the number of fatalities, and used death and infection percentages taken from 1,300 people placed on repatriation flights in China to extrapolate the percentage of infected people in the United States.

The death rate of 0.8% commonly used by researchers was calculated from the studies done on the repatriation flights. That’s eight times worse than the flu.

The calculations took into account the three to four weeks it usually takes the sickest people to die and the average speed of the virus’ spread and factored in the shelter-in-place and other measures taken to reduce the infection rate. Out of that came estimates of the total number of infections — known and unknown — in every state.

The model estimated 9 million infections in New York state. That’s 46% of the state’s 19.5 million residents. Second worse was New Jersey, where the model said 1.4 million people were infected, or 16% of the population. Louisiana was third with 10% of its population projected to be carrying the virus.

“The estimated value in New York is so high that it is likely close to herd immunity,” Thorpe and Lyberger wrote in the report, referring to the level at which so many people are infected that the spread slows because so many people are immune.

No other state is close to herd immunity, the study said.

The 270,000 people estimated to be infected in California represent 0.69% of its 39 million residents. As of Tuesday, 25,367 positive cases had been reported in California, including 766 deaths. In the Bay Area, there were 5,307 reported cases and 146 deaths.

Overall, 4.8% of the U.S. population was carrying the disease by early April, according to the model. Excluding New York state, which accounts for half the estimated infections in the country, the infection rate is 2% of the population, according to the calculations.

As of Tuesday, there were 592,743 reported cases in the United States, according to Johns Hopkins University.

The huge number of undocumented infections outlined in Thorpe and Lyberger’s study could explain the rapid geographic spread of SARS-CoV-2 — the virus’ formal name — and indicate how challenging containment of this virus will be. But like some of the previous studies, the projections used statistics from a limited number of cases in China, which experts say could be problematic.

Joseph Lewnard, an assistant professor of epidemiology at the UC Berkeley School of Public Health who uses mathematical and statistical modeling to study infectious diseases, said the study doesn’t pass the “smell test.”

“Even if the situation seems to have come under control earlier in California, it is highly suspect that the cumulative infections would be roughly 100 times higher in New York than in California,” Lewnard said. “For a virus that first emerged in humans around December 2019 in China, it is very hard to picture that there have been enough generations of spread to result in 9 million infections in New York alone, given what we know about transmissibility.”

Thorpe, who works at a venture capital firm, Valor Equity Partners, said his study makes assumptions based on statistical analysis and, whether or not one agrees with the final tally, the point is crystal clear.

“The takeaway is there are a huge number of people infected — many more than have had positive test results,” Thorpe said. “It means the best thing for policymakers to do is exactly what we are doing in California. This is still spreading quite widely. If we were to relax these restrictions, the spread would start to accelerate again and you would see more cases and more fatalities.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite