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The number of Covid-19 cases in the United States may have been underestimated by up to 60%, researchers at the University of Washington have found.
The study, published in the Proceedings of the National Academy of Sciences, builds on research which found that the number of reported cases “is only a fraction of the total estimated number of infections.” This has important implications for the number of Americans who must be vaccinated to stop epidemics.
The document comes as a swathe of southern and midwestern states, particularly Arkansas, Missouri and Louisiana, are experiencing epidemics caused by Delta variant infections in unvaccinated people.
“There are all kinds of different data sources that we can draw on to understand the Covid-19 pandemic,” said Adrian Raftery, professor of sociology and statistics at the University of Washington and lead author of the study. .
But, he said, “Each data source has its own flaws that would give a biased picture of what’s really going on. What we wanted to do was develop a framework that corrects the flaws of multiple data sources and builds on their strengths to give us an idea of the prevalence of Covid-19 in a region, state or country in his outfit. “
The study incorporated data on deaths, the number of tests given each day and the proportion of people who come back positive. Importantly, it also incorporated data from studies of people randomly sampled for Covid-19 in Indiana and Ohio.
Random sample surveys provide strong evidence for the true prevalence of a disease, as they do not rely on people looking for tests, which often fail to detect asymptomatic infections.
Based on the analysis of this data, the researchers found that 65 million Americans may have been infected. Official counts put the number at around 33 million. Researchers at the University of Washington estimated that 60% of all cases were missed, with just one in 2.3 cases counted in Indiana and Ohio.
As of Monday, the number of Covid cases maintained by Johns Hopkins University in Baltimore and commonly mentioned by the media stood at nearly 34.5 million
Undercounts may “depend on the severity of the pandemic and the amount of testing in that state,” said Nicholas J Irons, study co-author and postdoctoral student.
“If you have a state with a severe pandemic but limited testing, the undercoverage can be very high and you miss the vast majority of infections that occur,” he said. “Or, you could have a situation where testing is rampant and the pandemic is not as bad. There, the undercoverage rate would be lower.
The findings have important implications for the prospect of achieving herd immunity, the point at which epidemics end because a virus cannot find new hosts. In May, scientists believed the herd immunity threshold for Covid-19 was around 80%, a number that has increased slightly with the emergence of highly contagious variants such as Delta.
By the spring of 2021, according to the study, around 20% of the American population had been infected with Covid-19.
Without a mass vaccination campaign, the results say, the United States is unlikely to achieve collective immunity anytime soon and therefore likely to experience waves of epidemics, hospitalizations and death.
Even with a mass vaccination campaign, the United States is unlikely to achieve herd immunity this year or perhaps never, due to the highly contagious variants, low vaccine acceptance in some states, and the fact that that children under 12 are not eligible for vaccines.
About 56% of Americans are fully vaccinated, but these inoculations are not evenly distributed. The northeastern states tend to have administered the highest proportion of vaccines, and the southern and midwestern states the lowest.
This uneven distribution allowed an outbreak of Delta infections to sweep unvaccinated people across Arkansas, Missouri and Louisiana. The vaccines currently licensed in the United States are very effective against the variants.
Low immunization rates, including among health workers, in turn sparked a national debate on immunization mandates, with hospital chains and major cities starting to institute mandates for workers to be immunized.
Also on Monday, the American Medical Association released a joint statement with dozens of groups of doctors and nurses, calling for mandatory vaccination of health workers. Elderly care workers, in particular, have one of the lowest vaccination rates in the medical industry. More than 40% have not yet received a vaccine.
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