Throughout March and April, Spain was one of the countries hardest-hit by the coronavirus.
The outbreak there has since slowed, but the country has recorded around 253,000 cases and 28,000 deaths so far – more than twice as many cases and three times as many deaths as Canada, according to data from Johns Hopkins University.
But even with all those cases, most of Spain’s 47 million people still remain vulnerable to the disease.
A study released this week found that only around five per cent of the Spanish population had antibodies showing they had encountered the virus. In Madrid, which had one of the biggest outbreaks, the number was only about 10 per cent. And what’s more, it looked like those antibodies faded away over a matter of months.
Experts say this is bad news for proponents of natural “herd immunity” – the idea that if enough people catch the virus, eventually, the population will be protected against future outbreaks.
“We’re very far away from achieving any level of immunity that would protect Spain against future outbreaks,” said Ashleigh Tuite, an infectious disease epidemiologist at the University of Toronto.
“I think it’s really a cautionary tale and a bit of a warning to other places that this is going to be with us for a while. And even in those places that have been worst hit, they’re still vulnerable to subsequent waves of infections.”
About ‘herd immunity’
Usually, Tuite said, the term “herd immunity” is used to describe vaccination coverage. As in, you need around 95 per cent of people to be vaccinated against measles for that group to be protected against future measles outbreaks.
You can also get immunity to a disease without being vaccinated, but it requires a lot of people to have caught it.
Dr. Nelson Lee, a professor at the University of Alberta who studies infectious disease, says for influenza, you start to get a little population protection at around 40 per cent immunity, but for real protection, it needs to be closer to 70 per cent of people having immunity.
“We actually do not know what level of herd immunity is required for COVID-19,” he said, adding, “There would be a huge price to pay” to get to a level similar to influenza’s.
“If you infect a lot of people in the community, there will be a lot of severe illnesses and deaths.”
Even in Sweden, which had higher infection rates because there were few restrictions at first, there has been little demonstrated immunity, he said, despite a very high death toll.
It’s also unclear exactly how the presence of antibodies, which most tests are looking for, translates into immunity from further infection, Tuite said.
“We don’t really know for sure,” she said. “Because it’s a new virus, we can look at the immune response and look at antibodies and try to infer how many people have been infected. But we don’t really know how long-lasting that immunity is.”
“I think a huge assumption with any sort of strategy that’s based on herd immunity is that immunity is long-lasting, which at this point we don’t entirely know.”
Ending the pandemic
The bottom line, Tuite thinks, is that it would be a bad idea to just let the virus run its course until enough people have been infected to be able to theoretically achieve herd immunity.
“Herd immunity may be an outcome that happens over the course of the pandemic, but it shouldn’t be a strategy.”
The infection curve wouldn’t just stop when 70 per cent of people were infected either, she said — the cases would just trend slowly downward over time.
And while the idea of allowing the virus to run its course was floated in the U.K. early in the pandemic, and countries like Sweden elected not to shut down to the same level as their neighbours, Tuite said she is not aware of any country actively pursuing herd immunity as a strategy right now.
However, parts of the U.S. might be testing this by default, she said. “There is a strategy of letting the virus spread without really strong focus or effort toward controlling it. Uncontrolled spread is sort of de facto saying, ‘Well, we’ll let this spread until we have immunity in the population.’”
For all these reasons, a commentary published alongside this week’s study on Spain called a strategy of herd immunity both “highly unethical” and “unachievable.”
It would be much better to end the pandemic with a vaccine, Lee thinks.
Until that happens, Tuite said, we should imitate countries that are keeping their outbreaks under control. “It’s testing, it’s contact tracing, it’s isolating people who are infected,” she said, along with personal-level interventions like physical distancing and mask use.
“I think at this point, it’s enough to keep the number of cases manageable and allow us to basically live with this virus until we have the vaccine.”