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Herd immunity won’t save us from the coronavirus pandemic

Herd immunity against COVID-19 may be off the table
WATCH: Herd immunity against COVID-19 may be off the table, experts say

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.

Read more: Only 5% of Spain’s population has coronavirus antibodies, despite severe outbreak, study finds

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.

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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.

Read more: Spain hits grim coronavirus record as Europe rushes to build hospitals

“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.

Read more: Measles outbreak — What parents need to know about herd immunity

You can also get immunity to a disease without being vaccinated, but it requires a lot of people to have caught it.

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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.

Coronavirus outbreak: Herd immunity ‘not a concept that should be supported’, Tam says
Coronavirus outbreak: Herd immunity ‘not a concept that should be supported’, Tam says

“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.

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“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.

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Read more: What is herd immunity and what does it mean for COVID-19?

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.

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Read more: Does letting kids get coronavirus help build immunity among Quebec’s wider society?

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.

Coronavirus outbreak: WHO says evidence suggests herd immunity hasn’t been achieved
Coronavirus outbreak: WHO says evidence suggests herd immunity hasn’t been achieved

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.’”

Read more: Legault pushes ‘herd immunity’ ahead of gradual reopening of schools

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With uncontrolled spread, we can expect a “substantial proportion [of people] would develop severe illness, and some of them will require hospitalization and die,” Lee said. There are also reports of people who have lasting damage, or long-term symptoms as a result of COVID-19.

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.

When will a COVID-19 vaccine be ready?
When will a COVID-19 vaccine be ready?

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.

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“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.”