One in five New Yorkers may have had Covid-19, antibody tests suggest

One of every five New York City residents tested positive for antibodies to the coronavirus, according to preliminary results described by governor Andrew Cuomo that suggested that the virus had spread far more widely than known.

If the pattern holds, the results from random testing of 3,000 people raised the tantalising prospect that many New Yorkers – as many as 2.7 million, the governor said on Thursday – who never knew they had been infected had already encountered the virus and survived. Cuomo also said that such wide infection might mean that the death rate was far lower than believed.

While the reliability of some early antibody tests has been widely questioned, researchers in New York have worked in recent weeks to develop and validate their own antibody tests, with federal approval. State officials believe that accurate antibody testing is seen as a critical tool to help determine when and how to begin restarting the economy and sending people back to work.

“The testing also can tell you the infection rate in the population – where it’s higher, where it’s lower – to inform you on a reopening strategy,” Cuomo said. “Then when you start reopening, you can watch that infection rate to see if it’s going up, and if it’s going up, slow down.”

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The testing in New York is among several efforts by public health officials around the country to determine how many people may have been already exposed to the virus, beyond those who have tested positive. The results appear to conform with research from Northeastern University that indicated that the coronavirus was circulating by early February in the New York area and other major cities.

In California, a study using antibody testing found rates of exposure as high as 4 per cent in Santa Clara County – higher than those indicated by infection tests, though not nearly as high as found in New York. Public health officials recently disclosed that a woman in Santa Clara who died February 6th was infected with the virus.

In New York City, about 21 per cent tested positive for coronavirus antibodies during the state survey. The rate was about 17 per cent on Long Island, nearly 12 per cent in Westchester and Rockland counties, and less than 4 per cent in the rest of the state.

State researchers sampled blood from the approximately 3,000 people they had tested over two days, including about 1,300 in New York City, at grocery and big-box stores. The results were sent to the state's Wadsworth facility in Albany, a respected public health lab.

Immunity

Dr Howard Zucker, the state health commissioner, said the lab had set a high bar for determining positive results, that it had been given blanket approval to develop coronavirus tests by the Food and Drug Administration and that state officials discussed this particular antibody test with the agency.

He said that while concerns about some tests on the market were valid, the state’s test was reliable enough to determine immunity – and, possibly, send people back to the office. “It is a way to say this person had the disease and they can go back into the workforce,” Zucker said. “A strong test like we have can tell you that you have antibodies.”

But he cautioned that the length of any such immunity remained unknown. “The amount of time, we need to see. We don’t know that yet,” he said, adding, “They will last a while.”

Unlike so-called diagnostic tests, which determine whether someone is infected, often using nasal swabs, blood tests for Covid-19 antibodies are intended to reveal whether a person was previously exposed and has developed an immune response. Some tests also measure the amount of antibodies present.

Hours before Cuomo’s presentation, a top health official in New York City expressed general scepticism about the utility of antibody tests – especially those on the private market – when it comes to questions of immunity and critical decisions over social distancing and reopening the economy.

Dr Demetre Daskalakis, the city's top official for disease control, wrote in an email alert Wednesday that such tests "may produce false negative or false positive results," pointing to "significant voids" in using the science to pinpoint immunity.

The alert, sent to medical providers and other subscribers, went on to warn that the consequences of relying on potentially false results may lead to “providing patients incorrect guidance on preventive interventions like physical distancing or protective equipment.”

Health experts have worried that if tests return high rates of false positives, they could encourage people to abandon protective measures and risk worsening the virus’s spread. Others warn that the true value of coronavirus antibodies is still unknown.

Medical decisions

The World Health Organisation, a United Nations agency, recommends that antibody testing be used only in research settings and not to make medical decisions, such as to permit an individual to return to work. States and the federal government are not bound by the organisation's advice.

"I'm very ambivalent about these tests because we don't really know yet through the science what it means to have an antibody," said Dr Joan Cangiarella, vice chair of clinical operations at NYU Langone Health's pathology department.

“We are hoping these antibodies mean you will be immune for some time,” she said. “But I don’t think the data is fully out there to understand if that means that you’re actually immune. And if these antibodies start to decline, what’s that time frame? Does it decline in a year from now?”

Cuomo on Thursday did not talk about any potential for immunity among those previously infected. But he did suggest, based on the survey, that if as many as 2.7 million New Yorkers had the virus, the death rate in New York from Covid-19 would most likely be far lower than previously believed, possibly 0.5per cent of those infected.

More than 15,000 people have died of the virus in the state, a figure that does not include an additional 5,000 people in New York City who were never tested but were presumed to have died from the disease. The number of deaths has been increasing less quickly, and new hospital admissions for the coronavirus have remained relatively flat over the past three days: about 1,350 patients per day, down from more than 2,000 per day last week. More than 263,000 have tested positive for the infection.

Cuomo said antibody testing results, along with hospitalisation numbers, would influence the state’s reopening strategy, noting that the number of people being hospitalised was still too high to consider easing restrictions. The state’s plan would involve tracking infections as restrictions are loosened on gatherings and businesses.

Antibody testing would be used, Cuomo said, for identifying coronavirus survivors who can donate convalescent plasma – the part of the blood that contains antibodies. Doctors at the Mount Sinai Health System in New York and elsewhere have been testing this use of plasma for treating patients with the virus. – New York Times