A serological survey suggests COVID was circulating in New York City well before the first confirmed case was detected, and that 20% of the city's population has already been infected with the virus.
The first case of COVID-19 was reported at Mount Sinai Hospital in New York on February 29, 2020. Following that, the city experienced a rapid rise in infections and subsequent deaths. New York State issued a stay-at-home order on March 22, after which the number of new cases in the city began to decline in April and May.
By analyzing more than 10,000 plasma samples obtained from patients at Mount Sinai between early February and early July, researchers pieced together when anti-SARS-CoV-2 arose among two different groups of patients, those seeking emergency care and those undergoing routine care. As the Mount Sinai team reported on Wednesday in the journal Nature, their testing uncovered seropositive samples from as early as mid-February, suggesting the virus was circulating in New York earlier than thought.
"We now know there were many asymptomatic and mild-to-moderate cases that likely went undetected," study author Emilia Mia Sordillo from the Icahn School of Medicine said in a statement. "In this study, we aimed to understand the dynamics of infection in the general population and in people seeking urgent care."
The researchers collected de-identified plasma samples from patients seen in the emergency room or admitted for urgent care between February 9 and July 5. The urgent care group of 4,101 samples served as the positive control group in which the researchers expected to detect increasing numbers of SARS-CoV-2 infections as patients sought care.
At the same time, the researchers collected samples from what they called the routine care group, which encompassed OB/GYN or labor and delivery visits, cancer treatments, elective surgeries, and other office visits. This group acted as a proxy for the wider city population, as these patients were not seeking COVID-19-related care.
The researchers used a two-step ELISA test developed at Mount Sinai that detects antibodies to SARS-CoV-2 in the sample and determines the antibody titer, with sensitivity of 95% and specificity of 100%.
Within the urgent care group, no seropositive results were detected among the early February samples. Some seropositive results arose in mid-to-late February before sharply increasing toward the end of March. Seroprevalence in this group peaked in mid-April, before settling at just more than 20%.
The routine care group, meanwhile, had low seroprevalence between early February and mid-March, before it increased in late March and then stabilized at about 20%.
The first seropositive sample from this survey dated to the week of February 23 — before the first confirmed SARS-CoV-2 case in New York City. This suggested that the virus was likely introduced to the city a number of weeks earlier than thought.
With a seroprevalence of 22 percent, the researchers estimated that at least 1.7 million New Yorkers may have been infected with SARS-CoV-2 thus far. Still, they noted that the seroprevalence in the city falls below the expected threshold for potential community immunity, estimated to be about 67%. Additionally, they reported that the antibody titers among their cohort remained stable between May and July.
Using these numbers, the researchers calculated an infection fatality rate of 0.97% for SARS-CoV-2 — a rate they noted is much higher than the rate of between 0.01% and 0.001% calculated for the 2009 H1N1 pandemic.
They added that they plan to continue this serological survey for at least a year. "Knowing the detailed dynamics of the seroprevalence shown in this study is important for modeling seroprevalence elsewhere in the country," senior author Florian Krammer from Mount Sinai said in a statement.
This story first appeared in our sister publication, Genomeweb.