Sweden's coronavirus deaths have dropped dramatically, but that doesn't mean its herd-immunity strategy worked, experts say

• Sweden's rate of coronavirus deaths per capita was among the highest in the world in May.
• Many experts attributed this to the country's decision to avoid a full lockdown and either deliberately or inadvertently pursue herd immunity.
• Sweden's deaths and ICU admissions have declined considerably since June, likely due to protections for nursing-home residents and more social distancing over the summer months.
• But cases could still climb again as schools reopen, nursing homes allow visitors, and people return home to cities after summer vacation.

Sweden seems to have staged a remarkable turnaround.

After imposing few lockdown rules relative to the rest of the world this spring, the nation's death count per capita soared to among the highest in the world in May. But over the last week, Sweden saw an average of just three daily deaths compared to a peak of 115 in April. Its death toll per capita — now around 58 deaths per every 100,000 people — has fallen behind that of 13 other countries, including the US and UK.

The country's daily ICU admissions have also hovered in the single digits since the beginning of July.

Experts attribute the decline in deaths to four main factors. One is Sweden's five-month ban on large gatherings, which likely helped slow transmission over time. Second is the country's official guidance asking people to social distance and work from home. Third is the country's summer vacation period, which led people to leave cities. And fourth is the nation's renewed focus on improving safety in nursing homes.

But Sweden's recent success doesn't mean its strategy worked overall.

"They basically reached this plateau now, whereas others reached it a few months ago," Christian Althaus, an epidemiologist at the University of Bern in Switzerland, told Business Insider. "Because they're lifting measures now, case numbers will probably go up."

Sweden lifted its ban on nursing-home visits last week. Children ages 17 and older went back to school in August. Starting October 1, Sweden is also expected to start allowing gatherings of up to 500 people at public events as long as physical distancing is observed.

Combined, experts say, those factors could increase the risk of new transmission, leading to a second wave.

What worked in Sweden

Residents of elderly care homes account for nearly half of Sweden's coronavirus deaths, so curbing transmission in those facilities in particular seems to have caused deaths to fall significantly.

Sweden banned visitors at nursing homes in March. Since then, the facilities have gotten stricter about requiring social distancing and face masks. In recent months, the Swedish government also began working with care homes to make it easier for staff to report instances in which an elderly patient didn't get proper care or sufficient access to a doctor.

In May, Sweden's prime minister pledged 2.2 billion kronor ($220 million) in funding for more staff in elderly care homes. By July, nursing-home staff in Stockholm said they were no longer short on personal protective equipment.

On top of these initiatives, timing seems to have been on Sweden's side.

The country's vacation period lasts longer than that of other countries: from May through September. During that time, many residents abandon cities in favor of lengthy holidays in the countryside, where people are more spread out. That likely people to spend more time outdoors, where transmission is less likely, and naturally decreased the frequency of interactions between people overall.

Softer measures for a longer period of time

While most countries were asking residents to stay home in March, Sweden's state epidemiologist, Anders Tegnell, controversially chose to keep Sweden's primary schools, restaurants, bars, and gyms open.

Earlier this month, Tegnell told the Financial Times that national lockdowns were like "using a hammer to kill a fly."

But he added that contrary to popular belief, Sweden's goal was never to allow the virus to run rampant until the majority of the population was exposed. Instead, Tegnell said, leaders relied on residents to exercise personal responsibility.

All but 2% of Swedish residents who responded to a government poll in April said they'd changed their behavior to protect themselves from COVID-19. In May, 87% of respondents reported that they were keeping a greater distance from others in shops, restaurants, and on public transport.

Sweden did impose some restrictions, though: It closed high schools and universities for three months, urged people to work from home, required social distancing in bars and restaurants, and told the sick and elderly to stay home. Gatherings of more than 50 people were banned in March.

"There is often quite a misconception about what has been done and what is being done in Sweden," Althaus said. "Whereas other European countries had maybe more strict measures and lockdowns, but came out of that in April or May, Sweden had softer measures, but kept them in place for a very long time."

Even so, the consequences of that lax approach are now clear: From January to June, Sweden recorded more than 51,000 deaths — its highest death toll over that six-month period since a famine swept the country 150 years ago.

"They misjudged quite a bit, obviously, because for a long time they were talking about 'Well this is just a bit like the flu or a bit stronger,'" Althaus said. "But you can also argue that the other European countries overdid it a bit. The right balance is probably in between."

Swedish's advantage: small households

In viewing Sweden's current low rate of coronavirus deaths, it's tempting to wonder whether other countries should take any queues. But experts say Sweden's small households make it hard to apply recent lessons learned there to other places.

"Before claiming 'what country X is doing would have [the] same effect here,' we need to consider whether there are key differences in population structure between countries," Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine, recently wrote on Twitter.

Importantly, Sweden has a lower proportion of multigenerational households relative to most other European countries. Roughly one-third of Sweden's elderly population lives alone compared to just one-fifth of elderly residents in Greece or Spain. Sweden also has one of the smallest average household sizes in Europe: about 2.2 people per household. By contrast, the US and Russia have an average of 2.6 people per household, while Brazil has 3.3.

Studies show that the coronavirus' secondary attack rate is highest among household contacts. A June study found that attack rate was 20% within households, compared to only 6% among the general public. Other studies have found that the attack rate among households could be as high as 50%. Althaus estimated that half of coronavirus transmission may occur within households.

"It can be helpful to think of an epidemic as a series of within-household outbreaks, linked by between household transmission," Kucharski wrote.

The high proportion of Swedish residents living alone or in small groups therefore likely made it easier to slow transmission — particularly after elderly-care facilities got their outbreaks under control.

'The death toll really came as a surprise'

Although Tegnell has changed his tune somewhat since the spring, emails obtained by Swedish journalists showed he initially considered whether Sweden could reach herd immunity — the threshold at which enough of a population becomes immune to the virus to halt transmission.

In April, Tegnell told the Financial Times that he expected 40% of people in Stockholm to be immune to the coronavirus by the end of May. That didn't happen: A study from University College London estimated that the level of infection in Stockholm was around 17% in April — the same as in London.

Then in June, Tegnell estimated that up to 30% of Sweden's population could be immune, but a national study showed that just 6.1% of people had developed coronavirus antibodies by late May.

Althaus said the idea that Sweden would reach herd immunity was "always sort of ridiculous." Scientists widely agree that the safest way to achieve herd immunity is through vaccination, not natural infection.

"This idea that, basically 50%, 60%, 70% of people get infected and then the problem is solved, that was never really based on scientific foundation," Althaus said. "It's very unlikely that something like that can be achieved, and even if it could be achieved, it would come — at least in countries with a population demography like European countries or the US — with a huge cost."

Tegnell has even admitted that Sweden should have implemented tougher restrictions.

"If we were to encounter the same illness with the same knowledge that we have today, I think our response would land somewhere in between what Sweden did and what the rest of the world has done," he told Swedish Radio in June.

Tegnell also told "The Daily Show" host Trevor Noah that he underestimated how hard the virus would hit older people.

"We calculated on more people being sick, but the death toll really came as a surprise to us," Tegnell said on the show in May, adding, "we really thought our elderly homes would be much better at keeping this disease outside of them."