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Juan Lama
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A cemetery in Manaus, Brazil, has cleared ground (right) for the graves of people killed by COVID-19, which has devastated the city. As much as two-thirds of the population of Manaus, a city of two million people in Brazil’s state of Amazonas, could have been infected with the new coronavirus. That’s a proportion high enough to have contributed to controlling the spread of the virus.Ester Sabino at the University of São Paulo, Brazil, and her colleagues searched for antibodies against SARS-CoV-2 in more than 6,000 blood samples collected by a Manaus blood bank between February and August (L. F. Buss et al. Preprint at medRxiv https://doi.org/ghcm6h; 2020). From the proportion of donors who tested positive for antibodies, the authors estimate that about 66% of the population had been infected by early August — months after the epidemic in Manaus peaked in May . The authors say that the high proportion of donors with antibodies to the virus suggests that Manaus might have reached ‘herd immunity’, the term for a scenario in which enough people are immune to an infection to control its spread. The team says its estimate accounts for several potential sources of bias, including false positives and false negatives in antibody testing. The findings have not yet been peer reviewed. Preprint available at medRxiv (Sept. 21, 2020): https://www.medrxiv.org/content/10.1101/2020.09.16.20194787v1
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Juan Lama
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Spain is one of the European countries most affected by the COVID-19 pandemic. Serological surveys are a valuable tool to assess the extent of the epidemic, given the existence of asymptomatic cases and little access to diagnostic tests. This nationwide population-based study aims to estimate the seroprevalence of SARS-CoV-2 infection in Spain at national and regional level. 35,883 households were selected from municipal rolls using two-stage random sampling stratified by province and municipality size, with all residents invited to participate. From April 27 to May 11, 2020, 61 075 participants (75·1% of all contacted individuals within selected households) answered a questionnaire on history of symptoms compatible with COVID-19 and risk factors, received a point-of-care antibody test, and, if agreed, donated a blood sample for additional testing with a chemiluminescent microparticle immunoassay. Prevalences of IgG antibodies were adjusted using sampling weights and post-stratification to allow for differences in non-response rates based on age group, sex, and census-tract income. Using results for both tests, we calculated a seroprevalence range maximizing either specificity (positive for both tests) or sensitivity (positive for either test). Seroprevalence was 5·0% (95% CI 4·7–5·4) by the point-of-care test and 4·6% (4·3–5·0) by immunoassay, with a specificity–sensitivity range of 3·7% (3·3–4·0; both tests positive) to 6·2% (5·8–6·6; either test positive), with no differences by sex and lower seroprevalence in children younger than 10 years (<3·1% by the point-of-care test). There was substantial geographical variability, with higher prevalence around Madrid (>10%) and lower in coastal areas (<3%). Seroprevalence among 195 participants with positive PCR more than 14 days before the study visit ranged from 87·6% (81·1–92·1; both tests positive) to 91·8% (86·3–95·3; either test positive). In 7273 individuals with anosmia or at least three symptoms, seroprevalence ranged from 15·3% (13·8–16·8) to 19·3% (17·7–21·0). Around a third of seropositive participants were asymptomatic, ranging from 21·9% (19·1–24·9) to 35·8% (33·1–38·5). Only 19·5% (16·3–23·2) of symptomatic participants who were seropositive by both the point-of-care test and immunoassay reported a previous PCR test. The majority of the Spanish population is seronegative to SARS-CoV-2 infection, even in hotspot areas. Most PCR-confirmed cases have detectable antibodies, but a substantial proportion of people with symptoms compatible with COVID-19 did not have a PCR test and at least a third of infections determined by serology were asymptomatic. These results emphasize the need for maintaining public health measures to avoid a new epidemic wave. Published in The Lancet (July 6, 2020): https://doi.org/10.1016/S0140-6736(20)31483-5
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Juan Lama
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In what may be the world’s most important math puzzle, researchers are trying to figure out how many people in a community must be immune before the coronavirus fades. We’ve known from the beginning how the end will arrive. Eventually, the coronavirus will be unable to find enough susceptible hosts to survive, fading out wherever it briefly emerges. To achieve so-called herd immunity — the point at which the virus can no longer spread because there are not enough vulnerable humans — scientists have suggested that perhaps 70 percent of a given population must be immune, through vaccination or because they survived the infection. Now some researchers are wrestling with a hopeful possibility. In interviews with The New York Times, more than a dozen scientists said that the threshold is likely to be much lower: just 50 percent, perhaps even less. If that’s true, then it may be possible to turn back the coronavirus more quickly than once thought. The new estimates result from complicated statistical modeling of the pandemic, and the models have all taken divergent approaches, yielding inconsistent estimates. It is not certain that any community in the world has enough residents now immune to the virus to resist a second wave. But in parts of New York, London and Mumbai, for example, it is not inconceivable that there is already substantial immunity to the coronavirus, scientists said. “I’m quite prepared to believe that there are pockets in New York City and London which have substantial immunity,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “What happens this winter will reflect that.” “The question of what it means for the population as a whole, however, is much more fraught,” he added. Herd immunity is calculated from the epidemic’s so-called reproductive number, R0, an indicator of how many people each infected person spreads the virus to. The initial calculations for the herd immunity threshold assumed that each community member had the same susceptibility to the virus and mixed randomly with everyone else in the community. “That doesn’t happen in real life,” said Dr. Saad Omer, director of the Yale Institute for Global Health. “Herd immunity could vary from group to group, and subpopulation to subpopulation,” and even by postal codes, he said. For example, a neighborhood of older people may have little contact with others but succumb to the virus quickly when they encounter it, whereas teenagers may bequeath the virus to dozens of contacts and yet stay healthy themselves. The virus moves slowly in suburban and rural areas, where people live far apart, but zips through cities and households thick with people. Once such real-world variations in density and demographics are accounted for, the estimates for herd immunity fall. Some researchers even suggested the figure may be in the range of 10 to 20 percent, but they were in the minority. Assuming the virus ferrets out the most outgoing and most susceptible in the first wave, immunity following a wave of infection is distributed more efficiently than with a vaccination campaign that seeks to protect everyone, said Tom Britton, a mathematician at Stockholm University. His model puts the threshold for herd immunity at 43 percent — that is, the virus cannot hang on in a community after that percentage of residents has been infected and recovered. Still, that means many residents of the community will have been sickened or have died, a high price to pay for herd immunity. And experts like Dr. Hanage cautioned that even a community that may have reached herd immunity cannot afford to be complacent....
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Juan Lama
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How many people have really been infected by the coronavirus? In one German town a preliminary answer is in: about 14%. The municipality of Gangelt, near the border with the Netherlands, was hard hit by covid-19 after a February carnival celebration drew thousands to the town, turning it into an accidental petri dish. Now, after searching blood from 500 residents for antibodies to the virus, scientists at a nearby university say they have determined that one in seven have been infected and are therefore “immune.” Some of those people would have had no symptoms at all. Their brief report (PDF), posted online in German, has big implications for how soon that town, and the rest of the world, can come out from lockdown. “To me it looks like we don’t yet have a large fraction of the population exposed,” says Nicholas Christakis, a doctor and social science researcher at Yale University. “They had carnivals and festivals, but only 14% are positive. That means there is a lot more to go even in a hard-hit part of Germany.” Here's why the true infection rate in a region matters: the bigger it is, the less pain still lies ahead. Eventually, when enough people are immune—maybe half to three-quarters of us—the virus won’t be able to spread further, a concept called herd immunity. But the German town isn’t close to that threshold yet, and to Christakis the preliminary figure is “unfortunate” because it means the virus still has more damage to do. The German report is among the first to survey a population for evidence of prior infection, data that scientists need to determine how far the pandemic has spread, what the real death rate is, and how many people show no symptoms at all. “It’s very preliminary, but it’s the kind of study we desperately need,” says Christakis, who believes the US should test as many as 200,000 people, from big cities like New York to small towns in the Midwest. “This is crucial to quantify a host of basic parameters.”
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