“On average, people with the coronavirus infect about two other people; most pass the virus to just one other person, or to no one else at all.
But some people go on to infect many more — often before they even get symptoms. Many of these transmission chains begin with superspreading events, where one person (usually in a crowded indoor space) passes the virus to dozens of others. Early contact tracing studies suggest these events have been a large driver of transmission around the world. By some estimates, 10 percent of people have been causing 80 percent of new infections.”
“To understand what might kick off a superspreading event, let’s review some basics about how this virus, SARS-CoV-2, spreads. Researchers have found that it often spreads through microscopic droplets created when an infected person coughs or sneezes — or even speaks — and another person breathes them in. These disease-containing droplets are a large part of the reasoning behind staying at least 6 feet away from people and wearing a mask in public.
But scientists are finding that the virus likely also spreads through even tinier, longer-lasting particles from breathing or speaking (or flushing a toilet) called aerosols. These are so small they can linger in the air after an infectious person has left — and may contain infectious virus particles for up to three hours. And they may be a key element to superspreading events: An infected person could seed a poorly ventilated indoor space with virus without even getting physically close to all the people they end up infecting.
Superspreading also appears to be more likely with SARS-CoV-2 because people typically have the highest level of the virus in their system (making them infectious) right before they develop symptoms. (This is very different from other severe coronaviruses like SARS and MERS, where people were most infectious seven to 10 days after they started feeling sick, when they were more likely to be in isolation or in medical care.) So thousands of people with active Covid-19 infections continue to go about their lives not knowing that they could be spreading the disease.”
“Some individuals seem to develop higher amounts of the virus in their system, upping their odds of transmitting it to others.
And given that the amount of virus in the body tends to shift over the duration of infection — rising until around the onset of symptoms, then declining — the chance that someone is a likely superspreader changes over time.”
“Herd immunity is the resistance to the spread of a contagious disease that results if a sufficiently high proportion of a population is immune to the illness. Some people are still susceptible, but they are surrounded by immune individuals, who serve as a barrier preventing the microbes from reaching them. You can achieve this through either mass infection or mass vaccination.”
“Most the evidence so far suggests that people who recover from a COVID-19 coronavirus infection do, at least for a time, develop immunity to the microbe. If that’s true, what is the disease-induced herd immunity threshold for the COVID-19 coronavirus? Various epidemiologists offer different answers, depending upon their estimates for the disease’s R0 and other variables, but most have converged on a threshold at around 60 to 70 percent.
More recently, some researchers have suggested that this threshold may be too high. In a new preprint, three mathematicians from Sweden and the United Kingdom, using an R0 of 2.5, calculate a reduction in the herd immunity threshold from 60 percent to 43 percent by incorporating some assumptions with respect to populations’ social activity levels and age structures.
A couple of new reports speculatively lower the possible herd immunity threshold for the coronavirus to just 10 to 20 percent of the population. This conjecture depends chiefly on assumptions about just how susceptible and connected members of the herd are.”
“There are no solid estimates for the percentage of the U.S. population that has already been infected by the coronavirus, but Youyang Gu and his team at COVID19-Projections estimate that right now the number is between 2.2 to 4.7 percent. That would mean that somewhere between 7.3 and 15.5 million Americans have been infected. A similar result emerges from a very rough calculation that multiplies the number of confirmed cases at 1.4 million by a 10-fold factor of undiagnosed cases and infections. (The 10-fold factor is derived from data recently reported by Indiana University researchers.)
The upshot: The U.S. as a whole is not yet close to achieving even the speculatively low estimate of the herd immunity threshold.”
“For years, urban planners have been singing the praises of population density.”
“”Density is a factor in this pandemic, as it has been in previous ones,” wrote Richard Florida in the CityLab website. “The very same clustering of people that makes our great cities more innovative and productive also makes them, and us, vulnerable to infectious disease.” Some big cities have handled the crisis better than others. Some rural areas have high infection rates, too. But, as an urban studies professor, he’s distressed at big-city vulnerability.”
“For six weeks behind the scenes, and now increasingly in public, Trump has undermined his administration’s own efforts to fight the coronavirus outbreak — resisting attempts to plan for worst-case scenarios, overturning a public-health plan upon request from political allies and repeating only the warnings that he chose to hear. Members of Congress have grilled top officials like Health and Human Services Secretary Alex Azar and Centers for Disease Control Director Robert Redfield over the government’s biggest mistake: failing to secure enough testing to head off a coronavirus outbreak in the United States. But many current and former Trump administration officials say the true management failure was Trump’s.”
““Interviews with 13 current and former officials, as well as individuals close to the White House, painted a picture of a president who rewards those underlings who tell him what he wants to hear while shunning those who deliver bad news.”