MIT Technology Review reports:

The large number of people already infected with the coronavirus in the US has begun to act as a brake on the spread of the disease in hard-hit states.

Millions of US residents have been infected by the virus that causes covid-19, and at least 160,000 are dead. One effect is that the pool of susceptible individuals has been depleted in many areas. After infection, it’s believed, people become immune (at least for months), so they don’t transmit the virus to others. This slows the pandemic down.

“I believe the substantial epidemics in Arizona, Florida and Texas will leave enough immunity to assist in keeping COVID-19 controlled,” Trevor Bedford, a pandemic analyst at the University of Washington, said on Friday, in a series of tweets. “However, this level of immunity is not compatible with a full return to societal behavior as existed before the pandemic.”

The exact extent to which acquired immunity is slowing the rate of transmission is unknown, but major questions like school reopening and air travel may eventually hinge on the answer.

What is known is that after rising at an alarming pace starting in May, new cases of covid-19 in Sun Belt states like Florida have started to fall. Some of that may be due to social distancing behavior, but rising rates of immunity are also a factor, according to Youyang Gu, a computer scientist whose Covid-19 Projections is among 34 pandemic models tracked by the US Centers for Disease Control and Prevention.

“Immunity may play a significant part in the regions that are declining,” says Gu. At least until the fall, which is how far his models look forward, he says, “I don’t think there is going to be another spike” of infections in southern states.

The US has been recording more than 1,000 covid-19 deaths and 45,000 confirmed cases a day. The flip side of the rapid spread, however, is there are progressively fewer vulnerable people to catch and spread the virus again. Researchers say they hope to determine how great a role the rise of this population immunity can play in managing the pandemic.

“Clearly, as susceptibility drops, disease spreading drops. No one can say different,” says Tom Britton, a statistician who models the pandemic at the University of Stockholm. “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.”

A virus outbreak will cease to grow, even without any control measures, when a threshold called herd immunity is achieved. That’s when so many people are immune that the virus can’t find new hosts quickly enough.

For the new coronavirus, the threshold for reaching herd immunity is unknown. Estimates vary widely: anywhere from 10% to 80% of the population might have to be infected, depending on how well the virus spreads but also on social factors like how much people ordinarily mix with one another. [Continue reading…]