Did Sweden Accidentally Blunder into COVID-19 Herd Immunity?

If so, that could be really good news for the rest of the world.

At the beginning of the COVID-19 pandemic, the Swedish government's chief epidemiologist, Anders Tegnell, denied that his relatively permissive approach to controlling the spread of the coronavirus was aimed at achieving herd immunity.

Unlike most other rich countries—including its Nordic neighbors Denmark, Norway, and Finland—Sweden did not institute a strict lockdown. The government did, in late March, ban public gatherings of more than 50 people, including at theaters and sporting events. But the country decided to let most bars, restaurants, primary schools, and retail shops stay open. Universities and high schools were closed, and people were urged to work from home if possible. Some social distancing rules were adopted, such as limiting the number of customers at a time in shops and providing only table service at bars and restaurants.

Herd immunity is the resistance to the spread of a contagious disease that results if a sufficiently high proportion of a population is immune to the illness. At that point, some people are still susceptible but they are surrounded by immune individuals, who serve as a barrier preventing the microbes from reaching them. Herd immunity can be achieved through either mass infection or mass vaccination. Epidemiologists have converged on an estimate that 60 to 70 percent of people need to either have been vaccinated or infected to reach herd immunity for COVID-19.

While not explicitly adopting disease-induced herd immunity as a policy goal, Swedish public health authorities evidently expected the coronavirus to run quickly through the country's population while not overwhelming its health care system. If a high enough percentage of Swedes became infected and recovered, then herd immunity would forestall a second wave of the disease.

At the end of April, Tegnell told CNBC: "In major parts of Sweden, around Stockholm, we have reached a plateau (in new cases) and we're already seeing the effect of herd immunity and in a few weeks' time we'll see even more of the effects of that. And in the rest of the country, the situation is stable." Also in late April, the Swedish Public Health Agency projected that 26 percent of Stockholm's 2 million residents would have been infected by May 1. "About 30 percent of people in Stockholm have reached a level of immunity," the Swedish ambassador to the U.S. told NPR. "We could reach herd immunity in the capital as early as next month."

Yet a May study of blood tests in Stockholm found that only 7.3 percent of the city's residents had produced antibodies in response to being infected by the coronavirus. This suggested that country was still far from that 60 to 70 percent threshold.

On June 3, as COVID-19 cases in Sweden continued to mount, Tegnell told Swedish Radio, "Should we encounter the same disease, with exactly what we know about it today, I think we would land midway between what Sweden did and what the rest of the world did." In late April, daily Swedish COVID-19 deaths did peak, but it took until late June for the daily number of cases to begin to decline.

By late July, Tegnell was saying, "The epidemic is now being slowed down, in a way that I think few of us would have believed a week or so ago." He added, "It really is yet another sign that the Swedish strategy is working. It is possible to slow contagion fast with the measures we are taking in Sweden." On August 8, the Financial Times quoted Tegnell arguing that "there is a relationship between the very quick drop of the last few weeks and the increasing immunity in many parts of Sweden."

Noting the recent drop in the daily tally of COVID-19 cases, Tegnell observed on August 9, "Exactly why this happened at that time and why it was so quick and sudden, is difficult for us to understand." Tegnell acknowledges that the results of antibody blood tests do not find that enough Swedes have been infected and recovered to confer herd immunity as would be conventionally expected by epidemiologists.

So what could explain the "quick and sudden" drop in Sweden's COVID-19 case and death rates? This is very speculative, but Swedish public health authorities may have accidentally blundered into herd immunity through a combination of previously unsuspected extensive pre-existing T-cell immunity to the coronavirus and differential risks of infection due to social interaction variations among its people.

First let's look at T-cell immunity. Recent research suggests that people who have been infected with the milder coronaviruses that cause the common cold also have developed some immunity to the COVID-19 virus.

Two studies published in June—one by researchers associated with Karolinska Institute in Sweden, the other by researchers at the University Hospital Tübingen in Germany—identified coronavirus reactive T-cells, which are another virus-fighting component of the immune system. Earlier this month, a study in Science noted that T-cells that react to and counter COVID-19 coronavirus infections have been extensively reported in unexposed individuals, suggesting a pre-existing immune response in 20 to 50 percent of the population. So pre-existing T-cell coronavirus immunity among a significant proportion of the population may now be functioning as a barrier to COVID-19 infections, thus contributing to that 60 to 70 percent threshold.

Now let's examine how age cohorts and social interaction propensities might affect the threshold. Science just published a modeling study that calculates, based on various assumptions about population age structures and social contact rates, that the herd immunity threshold for COVID-19 could be as low as 35 percent.

In a July 24 preprint study, a team of researchers associated with Oxford University's Centre for Tropical Medicine and Global Health estimate that the COVID-19 herd immunity threshold could be as low as 10 to 20 percent. In their calculations, the team assumes that the individuals who are more susceptible or more exposed tend to become quickly infected and thus become immune early in the epidemic. Their subsequent interactions with the still-uninfected will thus not result in additional cases. Basically, they think the virus may stymie itself by disproportionately removing those most useful to it from contributing to its future transmission.

The Oxford researchers apply their analysis to the United Kingdom, Belgium, Spain, and Portugal. They conclude that all four countries, will reach a herd immunity threshold

between July and October and the COVID-19 epidemic [will be] mostly resolved by the end of 2020. Looking back, we conclude that [non-pharmaceutical interventions] had a crucial role in halting the growth of the initial wave between February and April. Although the most extreme lockdown strategies may not be sustainable for longer than a month or two, they proved effective at preventing overshoot, keeping cases within health system capacities, and may have done so without impairing the development of herd immunity.

You must keep firmly in mind the preliminary evidence for extensive pre-existing T-cell immunity and the speculative nature of these modeling studies. But combined, they do at least suggest the possibility that Sweden is more or less inadvertently close to achieving herd immunity—and, even better, that the end of the COVID-19 pandemic for much of the rest of the world may also be nigh.

How has Sweden fared in comparison with its Nordic neighbors that chose more comprehensive interventions? As of August 11, COVID-19 cases in Denmark, Norway, and Finland amounted so far to 14,959, 9,712, and 7,623, respectievly; deaths per million are at 107, 47, and 60, respectively. In Sweden, cases stood at 83,126 and deaths per million at 571. In the U.S., meanwhile, cases totaled 5,265,034 with deaths per million were at 503.

How did Sweden's less vigorous adoption of non-pharmaceutical interventions affect its economy? Sweden's GDP dropped 8.6 percent in the second quarter, compared to the European Union average of 11.9 percent. On the other hand, Sweden's unemployment rate rose to 9.8 percent in June, while the European Union's overall unemployment rate is 7.1 percent. And Sweden's neighbors? Denmark's unemployment rate is 5.5 percent; Norway's is 4.6 percent; Finland's is 7.9 percent. Meanwhile, the U.S economy shrank in the second quarter by 9.5 percent, and our an unemployment rate at the end of July stood at 10 percent.