There are only two ways to innoculate a population against the threat of a novel pathogen: vaccination or herd immunity. Vaccination is simply not an option for the coronavirus at the moment (nor for the next 12-18 months), so that leaves us with herd immunity — the notion that a viral infection stops spreading in a community after a sufficient proportion develops antibodies after recovering from the illness.

Although no country has officially declared herd immunity an objective they’re actively pursuing as part of their COVID-19 mitigations strategies, some have flirted with this idea. For instance, Sweden has taken a much more relaxed approach compared to its Scandinavian neighbors, or the world at large for that matter. In the scientific community, this approach (along with the UK’s initial approach) has been seen been by outsiders as a sort of massive (and very dangerous) medical experiment.

While residents in most countries stayed cooped up inside under strict orders not to leave their homes unless necessary, Swedes were told to behave responsibly and far fewer restrictions were truly imposed.

Schools and cafes were allowed to stay open and gatherings of up to 50 people are still fair game. Apart from minor restrictions, the life of the average Swede hasn’t changed all that drastically — at least not in terms of restricted movement.

Unsurprisingly, infection rates skyrocketed compared to neighboring lockdown-loving countries, such as Denmark or Norway.

As of June 1, Sweden had 37,532 cases and 4,395 deaths due to COVID-19, whereas Denmark had 11,699 cases and 576 deaths, and Norway 8,446 cases and 236 deaths. Sweden has a population roughly twice that of Denmark or Norway but its number of cases per million citizens is much higher.

This approach has caused Swedish authorities to come under a lot of fire, but is the country at least close to herd immunity? Not even close, according to recent assessments.

Herd immunity is a distant dream and quite possibly a disastrous strategy to follow

When our body’s immune system is attacked by a virus, white blood cells create antibodies to fight off the invading infection. When the virus comes in contact with us a second time, this time the immune system is primed against it, eliminating the infection before we start to feel sick.

Researchers conducted seroprevalence surveys, in which they took blood samples and analyzed them for antibodies against SARS-CoV2. The samples were taken randomly across a representative sample of the population.

In Stockholm, the most densely populated region of the country, just 7.3% of the population is estimated to have antibodies to the coronavirus. Elsewhere in the country, it’s likely that a much lower proportion of the population has the antibodies.

Elsewhere, in Spain, a similar survey reported a national average of 5% with antibodies for COVID-19, with 11% for Madrid and 7% for Barcelona. In the UK, the national average for COVID-19 antibodies is 5%, although London has 17%.

Worldwide, the WHO estimates that just 3% of the population has antibodies to COVID-19, almost half a year since the outbreak first started in Wuhan, China.

In order for herd immunity to offer protection from infection to a population, at least 60% of the community would have to have the required antibodies.

Herd immunity: not a viable strategy in light of insufficient information about the coronavirus

Herd immunity hinges on two important factors: the proportion of the population that is infected and the duration of immunity.

In the case of SARS-CoV-2, the question of how long immunity lasts after recovery is still open. In April, the WHO issued a statement claiming there is “no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection.”

But since then, research has surfaced suggesting that people do indeed form antibodies and that SARS-CoV-2 isn’t necessarily some superbug on course to rewrite immunology textbooks. One recent study on 285 people who tested positive for Covid-19 found that all of them developed antibodies within 19 days of symptom onset.

As for the question of how long immunity lasts, that’s impossible to tell at this point because the virus is still so new. We’ll have to wait at least a year for definite answers. If immunity only lasts for a few months (which is the case with influenza, for instance), that means that even the people that are immunized now might not be immunized next year, so this makes the herd immunity strategy even more far-fetched.

However, SARS-CoV-2 isn’t the only coronavirus that we know of. Exposure to MERS-CoV, which caused the Middle East respiratory syndrome (MERS) outbreak in 2002-2003, produced antibodies that were present 2-3 years post-illness, according to one study. Another 2016 study found that SARS-recovering patients had T-cells capable of fighting the virus even 11 ears post-infection.

The bad news is that coronaviruses that cause the common cold frequently reinfected previous hosts more than once over the span of a year, indicating that immunity is very short-lived. If immunity to SARS-CoV-2 is anything like that for the cold, then herd immunity won’t work. As such, any national strategy hinging on herd immunity should first wait for peer-reviewed studies that establish how long antibodies for COVID-19 stay in a person’s blood.

But even if immunity would be somewhat long-lasting, more than 57% of the global population would have to be infected, potentially resulting in millions of deaths. While we wait for a vaccine, it seems like the only viable option at this point is suppression/mitigation through constant testing, fast contact tracing as soon as new clusters are identified, and social distancing for many months to come.