About 6 percent of Miami-Dade’s population — about 165,000 residents — have antibodies indicating a past infection by the novel coronavirus, dwarfing the state health department’s tally of about 10,600 cases, according to preliminary study results announced by University of Miami researchers Friday.
The study, spurred by Miami-Dade County officials, will be an ongoing weekly survey based on antibody testing — randomly selecting county residents to volunteer pinpricks of their blood to be screened for signs of a past COVID-19 infection, whether they had tested positive for the virus in the past or not. The goal is to measure the extent of infection in the community.
Friday’s results, based on two weeks of countywide antibody testing and about 1,400 participants, found that about half of the people who tested positive for antibodies reported no symptoms in the 14-17 days before being tested. If the trend holds, the findings could have major implications for understanding not only the number of people infected, but also how many have symptoms and, in turn, how the virus spreads.
Erin Kobetz, a University of Miami professor of medicine and public health sciences and the lead researcher on the project, presented the findings along with Miami-Dade Mayor Carlos Gimenez during a press conference on Friday.
Gimenez said the survey’s early findings are informing the county’s decisions about how to allow limited openings of public areas, such as county parks. But he emphasized that social distancing measures will remain in place — and enforced by police and an army of 400 “event staff” members that Miami-Dade hired to ensure people follow the rules.
“Whatever measures we’re going to do to open up, especially the outdoor spaces, is going to be done with those measures in mind,” he said. “You’re not going to be able to get close. You’re going to have social distancing.”
Gimenez said the UM survey of Miami-Dade reflects a national trend showing African Americans are disproportionately impacted by the novel coronavirus. “African Americans are more than twice as likely to be infected with COVID-19 than other racial groups,” he said.
The survey findings also indicate that Miami-Dade is far from the 60% or higher infection rate needed to reach herd immunity, when enough people are infected to stop the spread of the virus by slowing transmission.
As scientists and policy makers struggle with the question of when it will be safe for businesses to reopen and people to return to work, blood tests like the ones used by UM researchers, called serological tests, will be one of the main methods to help formulate those plans, said Michael Mina, an assistant professor of epidemiology at Harvard T.H. Chan School of Public Health.
“That’s really why there’s been so much interest in getting serological evaluations performed,” he said. “It’s the huge missing piece from this epidemic.”
Community-wide serology tests will also help doctors and researchers learn more about the body’s immunological response to the novel coronavirus, especially about how long immunity lasts and what specific antibody level conveys immunity. UM’s testing only detects the presence of antibodies and does not measure the level, Kobetz said.
UM researchers used statistical methods to account for the limitations of the antibody test, which is known to generate some false positive results. The researchers say they are 95% certain that the true amount of infection lies between 4.4% and 7.9% of the population, with 6% representing the best estimate.
That would mean about 165,000 estimated infections in Miami-Dade, with the margin of error equating to 123,000 residents on the low end and 221,000 residents on the high end.
The results are similar but not identical to other serological surveys in California, but indicate far less infections than a recent survey in New York, which found a nearly 14% infection rate statewide and upwards of 20% in New York City.
Mina, the Harvard epidemiologist, said infection rates based on tests done when the virus is active — the most common form of testing to date — likely under-represent the true extent of the pandemic about ten-fold.
But Mina cautioned that the infection rate is likely to be higher in densely populated communities than rural areas. “This is not a virus that has homogeneous spread,” he said. “This is a virus that has clusters of really, really high infection rates and then there will be areas where it’s just not so much.”
UM researchers say their findings are more robust than most because they used Florida Power & Light to generate phone numbers in targeted demographic areas, leading to a more randomized selection of participants.
Results from a serological survey in Santa Clara County, California, released last week, estimated between 2.5% and 4.2% of the population was infected, but the survey came under widespread criticism for, among other limitations, recruiting participants through Facebook ads, which scientists say could skew results by including people who think they got sick and want to confirm their suspicions.
That survey found an infection rate that would be 50 to 85 times higher than the official number of confirmed cases, while UM’s more randomized study indicates an infection rate about 15 times higher than the official case count.
Natalie Dean, a biostatistician and assistant professor at the University of Florida, has been following serological surveys across the country. She said UM’s preliminary results show a level of under-detection that is roughly consistent with the New York results and other estimates — aside from the more controversial Santa Clara study.
“I would expect this factor to be somewhat stable across the U.S., although it will vary due to differences in testing capacity,” Dean said.
Kobetz told the Miami Herald she welcomed criticism of the study’s limitations, which is how science advances knowledge as a field, but felt it was necessary to move research forward in the midst of a pandemic, “given the constraints of a rapidly evolving situation where people are sick and at risk of dying.”
The UM study will expand to voluntary testing in hard-hit areas and will follow certain participants to measure their antibodies over time, Kobetz added. Given the diverse demographics of Miami-Dade County, Kobetz said the study could offer unique insights into how the virus spreads and its impact on certain populations.
“Miami-Dade County looks like the future of this country,” Kobetz said. “Our ability to understand infection dynamics here matters.”