Population immunity is slowing down the pandemic in parts of the US

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.

Herd immunity

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.

Once an obscure inflection point known only to epidemiologists, herd immunity has gained what Francois Balloux, a systems biologist at University College London, calls Kardashian-like" fame and become a lightning rod in politically polarized debates over economic reopening. On social media, some commenters insist that herd immunity has already arrived, meaning lockdowns and school closures are not necessary. Others find reason to doubt that immunity will ever accumulate without a vaccine and say counting on it can only lead to millions of deaths.

It seems there is the herd immunity is already reached' team and the we are all going to die' team. The good thing is, that there is a third let's get the data and let's look at what this all means team' out there," tweeted Florian Krammer, an immunologist at the Icahn School of Medicine in New York City.

What is certain is that in the US, with a raging epidemic, natural immunity is building fast. During June and July, Gu estimates, 450,000 people a day were being infected by the coronavirus in the US, the highest figures since the disease arrived in February.

That number is higher than the official case count because it includes an estimate of infections that go unseen, unfelt, or unreported. In June, CDC director Robert Redfield told reporters that the real number of infections could be many times the official tally. For instance, Gu has estimated that about 35 million Americans have now been infected-roughly 10% of the nation's population.

Natural infection also turns out to be extremely efficient at reducing virus transmission-even more effective than an equal number of people getting a vaccine. The reason is that the virus has been finding and infecting precisely those people who-whether because of behavior, circumstances, or biology-are most likely to be part of transmission chains.

Perhaps they are college students on spring break, or hospital nurses, or people who touch their face all the time. Whatever the reason, once these individuals become infected and are removed from the equation through death or immunity, the effect on the pandemic is outsized. By contrast, vaccinating a sheltered older person might protect that individual but does relatively less to stop transmission.

When the disease itself causes herd immunity, it does so more efficiently than when we give out vaccine at random," Marc Lipsitch, a public health modeler at Harvard University, told the political pundit Bill Kristol last week during a podcast interview. As a result, there is discussion" about whether viral transmission could be reduced more quickly than generally believed, he says.

Lessons from Sweden

Outside the US, researchers are also closely tracking the role of population immunity in national responses. Sweden, for example, did not impose a strict lockdown, and saw a large number of deaths starting in April. Since then, however, the number of new infections has declined. The nation's leaders said last week that children would go back to school unmasked.

I would say in Sweden there is no doubt that immunity plays an important role, more than in other countries," says Britton. Now this epidemic is slowly stopping."

Britton says a better understanding of how population immunity is shaping outbreaks could help guide the level and intensity of social interventions. He says the goal is to keep the virus's reproduction number, called R, below 1, meaning every person with the virus infects fewer than one other. Under those conditions, an outbreak dwindles.

Herd immunity is when all restrictions can be relaxed and you are still below 1," he says. But immunity doesn't have to be at that level to have an effect."

In some cities, like New York and Miami, blood tests show that 20% or more of the population has had the virus. But in regions still little affected, like small towns or rural areas, the population remains more susceptible. That means the virus can and will continue to cause new outbreaks. For instance, Louisiana saw a large spike in infections, followed by a lull and then a second spike. This occurred as the virus first hit New Orleans and later reached the rest of the state.

The geographic unevenness of the pandemic is one reason Britton does not think Sweden is able to get back to normal yet. Are we protected from big outbreaks if all the restrictions are released? The answer is no," he says. On a national scale the immunity is not that high-it might be 20%. But in Stockholm it's maybe 30 or 40%. We may be close to herd immunity [there], so they could relax restrictions a bit more."