Cases of have been high in the East Cambridge and Wellington-Harrington ZIP code, even as counts surge citywide. (Photo: Marc Levy)

The number of Cambridge residents who have died from Covid-19 rose sharply Thursday, increasing to 53 from 45, or 18 percent. All but two new deaths were in nursing home residents, who account for 79 percent of deaths from the virus.

The number of deaths in the nursing home population increased to 42 from 36, the city public health department reported. In those outside nursing homes it rose by two. Total cases of coronavirus stood at 758 on Thursday, up 20 cases. For the first time since a second round of testing for all nursing home residents was completed two weeks ago, the number of infections inside the facilities increased markedly, from 240 to 251.

Nursing home residents are especially vulnerable to severe illness from Covid-19 because of their age and poor health. Infections can spread rapidly when people don’t have their own living space and employees who may be infected themselves move from one resident to another. In many facilities with outbreaks, the number of deaths has spiraled quickly.

All three Cambridge nursing homes have had outbreaks of more than 30 cases, according to data from the state Department of Public Health. None of the facilities have disclosed how many residents are infected, and the Cambridge Public Health Department hasn’t provided any details.

Race and nursing homes

Still, the department is providing more and more other data about Covid-19 in Cambridge. This week the city started reporting case numbers by race and ethnicity as well as case rates – the number of cases per 10,000 population for that racial or ethnic group.

Those case rates bore out Chief Public Health Officer Claude Jacob’s statement to the City Council on Monday that black residents are getting Covid-19 at three times the rate of white residents. The rate for blacks was 125 cases per 10,000 population, compared with 43 cases per 10,000 for whites, according to Thursday’s report.

As Jacob said, black residents are more likely than whites to work in frontline jobs that could expose them to the virus, have less access to medical care and have lower income. Jacob also said the department would not include nursing home residents in the case rate data because they are overwhelmingly white and would skew the comparisons.

But the city is including nursing home residents, and it would be difficult to exclude them, since there are no population estimates for racial and ethnic groups that don’t include nursing home residents.

Students and ZIP codes

Besides that, the city’s notes on its racial data say that most of Cambridge’s large contingent of university students left the city when the universities closed their campuses in response to Covid-19, but population estimates used in calculating the case rates don’t reflect that. The students left before the first case of coronavirus in Cambridge was reported.

More than 23,000 college students usually live in Cambridge, according to the latest reports from Harvard, the Massachusetts Institute of Technology, Lesley University and other institutions. More than 11,000 are graduate students, and some might still be here.

The higher population estimates would tend to decrease the case rate. That might explain why the case rate for Hispanic residents is slightly lower than the rate for whites; the Asian case rate is also lower.

The notes say the fact that students are gone while they remain in the population estimates might decrease the case rates artificially for ZIP codes 02139, which includes the Massachusetts Institute of Technology, and 02138, which has Harvard and Lesley.

A map of the case rates by ZIP code has reported consistently that 02141, with East Cambridge and Wellington-Harrington, has the highest infection rate in the city. The department has not explained the finding or said what it is doing in response.