The number of Covid-19 cases across the US may have been undercounted by as much as 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 has found the number of reported cases “represents only a fraction of the estimated total number of infections”. It has important implications for how many Americans need to be vaccinated to stop outbreaks.
The paper comes as a swath of states across the south and midwest, especially Arkansas, Missouri and Louisiana, experience outbreaks driven by Delta variant infections among unvaccinated people.
“There are all sorts of different data sources we can draw on to understand the Covid-19 pandemic,” said Adrian Raftery, a professor of sociology and statistics at the University of Washington and senior study author.
But, he said, “each source of data has its own flaws that would give a biased picture of what’s really going on. What we wanted to do is to develop a framework that corrects the flaws in multiple data sources and draws on their strengths to give us an idea of Covid-19’s prevalence in a region, a state or the country as a whole.”
The study incorporated data on deaths, the number of tests administered each day and the proportion that 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 of actual prevalence of a disease because they do not rely on people seeking out tests, which often fail to capture asymptomatic infections.
Based on analysis of that data, researchers found as many as 65 million Americans may have been infected. Official tallies put the number at about 33 million. The University of Washington researchers estimated that 60% of all cases were missed, with only one in every 2.3 cases counted in Indiana and Ohio.
On Monday, the Covid case count maintained by Johns Hopkins University in Baltimore and commonly referred to by media outlets stood at nearly 34.5 million
Undercounts can “depend on the severity of the pandemic and the amount of testing in that state”, said Nicholas J Irons, a study co-author and postdoctoral student.
“If you have a state with severe pandemic but limited testing, the undercount can be very high and you’re missing the vast majority of infections that are occurring,” he said. “Or, you could have a situation where testing is widespread and the pandemic is not as severe. There, the undercount rate would be lower.”
The findings have important implications for the prospect of reaching herd immunity, the point at which outbreaks end because a virus cannot find new hosts. As of May, scientists believed the herd immunity threshold for Covid-19 to be around 80%, a number that has edged upward with the emergence of highly contagious variants such as Delta.
By the spring of 2021, the study indicates, about 20% of the US population had been infected with Covid-19.
Without a mass vaccination campaign, the findings indicate, the US would be unlikely to reach herd immunity any time soon and therefore likely to suffer waves of outbreaks, hospitalizations and deaths.
Even with a mass vaccination campaign, the US is unlikely to reach herd immunity this year or perhaps ever, because of highly contagious variants, low vaccine acceptance in some states and because children under 12 are not eligible for vaccines.
About 56% of Americans are fully vaccinated, but those inoculations are not evenly spread. North-eastern states tend to have administered the highest proportion of vaccines, and states in the south and midwest the lowest.
That uneven distribution has allowed an outbreak of Delta infections to sweep through unvaccinated people across Arkansas, Missouri and Louisiana. Vaccines currently authorized in the US are highly effective against variants.
Low vaccination rates, including among health workers, have in turn sparked a national debate on vaccine mandates, with hospital chains and major city governments beginning to institute mandates for workers to be vaccinated.
Also on Monday, the American Medical Association issued a joint statement with dozens of doctors’ and nurses’ groups, calling for mandatory vaccination of health workers. Eldercare workers in particular have among the lowest vaccination rates in the medical industry. More than 40% have yet to receive a shot.