The U.S. has confirmed more than 5.4 million cases of COVID-19 - although the true figure may be significantly higher - and recorded over 160,000 deaths, according to the Johns Hopkins Coronavirus Resource Center.
However, people may develop at least some form of immunity after infection, meaning the number of individuals who are vulnerable to the disease in hard-hit areas - such as southern states that have recently seen large spikes in infections - is dropping, according to pandemic analyst Trevor Bedford from the University of Washington.
"I wanted to discuss the degree to which population immunity may be contributing to curbing COVID-19 in Florida, Arizona and Texas, where recent surges have resulted in substantial epidemics," Bedford wrote in a series of tweets. "After increasing dramatically in June and July, daily case counts in Florida, Arizona and Texas have begun to subside."
"I believe the substantial epidemics in Arizona, Florida and Texas will leave enough immunity to assist in keeping COVID-19 controlled," he said.
Taking Florida as an example, Bedford estimated that around 20 percent of the state's population - around four million people - has had COVID-19, although it could "easily" be 10 percent.
"Assuming a large majority of infections leave enough immunity to be protected (which I believe to be the case) population immunity of 20 percent will have real impact if societal behavior has already reduced Rt to around 1.2," Bedford said.
Rt is the actual transmission rate of the virus at a given time. If it is more than one, the number of cases will keep rising. If it is lower than one, the number of cases will eventually peter out to zero.
"Even 10 percent population immunity starts to make a difference when Rt is around 1.2," Bedford said.
Scientists have proposed various estimates for when herd immunity against the novel coronavirus can be reached in a given population, with estimates ranging from as low as 10 percent infected to as high as 80. Many believe that herd immunity may only be achieved at an unacceptable cost of lives.
For a study published on Friday in the journal Science, researchers from Stockholm University in Sweden and the University of Nottingham in the U.K. decided to investigate the issue of herd immunity, finding that it could be achieved at a population-wide infection rate of around 40 percent - considerably lower than many previous estimates.
In their paper, the researchers focused on the fact that human populations are far from homogenous in terms of their age and activity levels. So they created a mathematical model to show how these differences might affect the spread of the disease.
They found that by introducing these differences into population models for the spread of the virus, the threshold for herd immunity could be reduced to around 40 percent because "the proportion of infected individuals in groups with the highest contact rates is greater than that in groups with low contact rates."
"This shift is because transmission and immunity are concentrated among the most active members of a population, who are often younger and less vulnerable," the researchers wrote. "If non-pharmaceutical interventions are very strict, no herd immunity is achieved, and infections will then resurge if they are eased too quickly."
Despite Bedford's estimates for increasing herd immunity in Florida, Arizona and Texas, the scientist warned that the level of protection he believes exists in these states is "not compatible" with a return to the kind of lives people lived before the pandemic.
Furthermore, Bedford said the costs to achieve this level of immunity have so far been "substantial" and continue to increase.
"I certainly believe that the 'herd immunity' strategy for dealing with COVID-19 is hugely overly costly in terms of health impacts. But it does seem like the strategy is being perhaps unintentionally pursued in parts of the U.S. We need a vaccine to achieve population immunity in a fashion that doesn't kill people."
It is still not clear to what extent any acquired immunity to the virus that causes COVID-19 - known as SARS-CoV-2 - is slowing down its spread. However, it is apparent that some states that saw significant spikes in new infections in June and July, such as Florida, are now starting to see new a reduction in new daily cases.
Youyang Gu, a computer scientist whose pandemic models are used by the Centers for Disease Control and Prevention, thinks that social distancing measures and behavioral changes may be responsible for some of the falls, while increasing immunity could also be playing a role.
"Immunity may play a significant part in the regions that are declining," Gu told MIT Technology Review. "I don't think there is going to be another spike" of infections in southern states," he said.
In fact, Gu estimates that around 35 million Americans have been infected with SARS-CoV-2, which is equivalent to around 10 percent of the population and far higher than the official number.
"Clearly, as susceptibility drops, disease spreading drops. No one can say different," Tom Britton, an author of the latest Science study from Stockholm University told the Technology Review. "The question is to what degree is the effect because of interventions or because of immunity? In regions with very large outbreaks - New York, Milan, Madrid, and London - I am convinced it's a combination."
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