TORONTO -- As politicians, policy makers and scientists struggle to find solutions and coping mechanisms for COVID-19 that balance the varying economic and health needs of the population, many ideas have been tossed around.

“Herd immunity,” is one such buzzword you may have heard, but what does it actually mean?

“Herd immunity is the concept that when enough people get an infection and are now immune, you’ve essentially protected the rest of the population because there’s nowhere for the virus to spread,” Dr. Christopher Labos, who has a degree in epidemiology, told CTV News Montreal.

“The virus has to spread from person to person to survive, so if you have enough people who are immune, the virus will eventually peter out.”

It’s an appealing concept in the time of COVID-19, as achieving herd immunity would mean that many amenities, workplaces and public areas could reopen because the risk of transmission would be so low.

The term was coined more than a century ago, according to a 2011 paper published in the scientific journal Clinical Infectious Diseases, but wasn’t used much until recent decades.

It has been used mostly in conjunction with discussions about vaccine production -- the idea is that if vaccines can be used to immunize enough of the population, once a certain threshold has been passed, even those who haven’t received a vaccine will be relatively safe because they will be surrounded by vaccinated individuals who can’t pass on the virus.

Doctors and medical experts frequently point to herd immunity as a reason why the anti-vaccine movement that became more popular over the past few years is dangerous -- herd immunity protects those who are too young to take vaccines from catching diseases.

However, there is no vaccine for COVID-19 yet, or even an effective treatment.

“The problem with (applying herd immunity to COVID-19) is, one, enough people have to actually get the virus and get sick, which results in a considerable loss of life in this particular case,” Labos said.

“The second problem is, we don’t know if immunity from the virus is going to stay long-term or not.”


Yes. And no -- while those who have fought off COVID-19 once definitely have a strong resistance to the virus, due to antibodies developed during their battle, not enough is known about the strength and duration of that immunity yet.

There have been media reports from several countries of individuals testing positive for COVID-19 after they had been thought clear of the virus. The World Health Organization (WHO) told several media outlets on April 11 that they were looking into reports of COVID-19 patients who tested positive a second time after testing negative while in consideration for being discharged.

Although these cases could be simply due to faulty tests, the possibility of patients being re-infected has not been ruled out. It’s also unknown if recovering from COVID-19 could grant immunity for a brief period, but then leave a person vulnerable again after some time -- in the case of regular influenza, a new flu shot is required every year.

Infectious disease specialist Dr. Abdu Sharkawy said in March that it’s “quite possible” for individuals who had COVID-19 to become infected again.

“One of the concerns we have is there may be a second wave of infection if our strategies for containment and for infection control are not good enough,” he told CTV’s Chief News Anchor and Senior Editor Lisa LaFlamme.

And while there may be tests to discover if a person has developed antibodies to fight against the virus, those tests still don’t prove 100 per cent that a person is fully immune, making it hard to track a population’s percentage of immunity.

An April 22 letter from Demetre Daskalakis, deputy health commissioner with NYC Health, cautioned against associating the presence of antibodies in a person with proof of full immunity.

The letter stated that serological tests -- which are being used to test for antibodies in people in order to track general trends of who has been exposed in the population, as well as find those who can donate plasma to CVOID-19 patients -- are not reliable enough to say for sure whether a specific individual is now immune to the virus.

“Although there is interest in identifying individuals who may be immune to SARS-CoV-2 due to previous infection, significant voids remain in our scientific understanding of the pathophysiology of SARS-CoV-2,” the letter stated.

In an interview this week with Good Morning America, Dr. Anthony Fauci, U.S. director of the National Institute of Allergy and Infectious Diseases, also said that antibody tests aren’t able to prove immunity.

“There’s an assumption — a reasonable assumption — that when you have an antibody that you are protected against reinfection, but that has not been proven for this particular virus. It’s true for other viruses,” he said.

Officials have emphasized multiple times that we are learning about the virus as we go. Without a proven vaccine, knowing who is fully safe from the virus is difficult.


Despite there being no vaccine in sight, the U.K. appeared early on to endorse this strategy in their response to the virus, with British officials saying in mid-March that they would conquer the virus through herd immunity.

As other countries across Europe and the world shuttered schools and restaurants, cancelled events and emphasized physical distancing to their populations, the U.K. was slow to restrict the everyday movements of its citizens.

The U.K.’s chief scientific advisor, Sir Patrick Vallance, appeared on Sky News on March 13 to defend the choice, saying they wanted to avoid a second, even larger peak in the winter if they did a full lockdown and then opened things up again.

He said they wanted to “allow enough of us, who are going to get mild illness, to become immune to this, to help with the sort of whole population response, which would protect everybody.”

For the U.K. to achieve herd immunity, Vallance said, 60 per cent of the population would need to contract the virus.

“Of course, we do face the prospect of … an increasing number of people dying,” he said.

Officials reversed course on this before the end of March after criticisms, instituting more restrictions on public gatherings before an effective nationwide lockdown was imposed on March 23.

More than 19,000 people have died of COVID-19 in the U.K., and there have been approximately 143,000 cases of the virus. With a population of more than 66 million, this gives them a death rate per capita of roughly 29 deaths per 100,000.

U.K. Prime Minister Boris Johnson also contracted COVID-19 and had to be admitted to an intensive care unit as a precaution, but eventually recovered.

Sweden is another country that has resisted lockdowns and closures of public spaces such as schools and restaurants.

In Stockholm, although gatherings over 50 are banned, it’s still possible to meet up with friends at a pub. The country has relied largely on individuals voluntarily isolating if they feel symptoms.

Sweden has less COVID-19 cases than many European countries, but it has vastly more when compared to its immediate Nordic neighbours. Norway has been in lockdown since mid-March, and Finland has closed schools and banned gatherings of more than 10 people. Both have seen a fraction of the deaths per capita that Sweden has.

Sweden, with 10.2 million people in total, has had more than 2,100 deaths, giving them a death rate per capita of 21 people per 100,000. Finland’s death rate is 3.2 people per 100,000, while Norway’s is 3.7 people per 100,000.

However, Sweden’s chief epidemiologist, Anders Tegnell, has not wavered in Sweden’s methods, and says they could reach herd immunity soon.

Tegnell told CNBC on Tuesday that they have reached a plateau in many parts of the country, and are “already seeing the effect of herd immunity.”

He said the high mortality rate in Sweden is due to deaths in long-term care homes, not because of a lack of lockdown measures, and that they are looking at how to better protect seniors.


When it comes to immunity, officials are also looking at which groups appear to be naturally less affected by the virus.

In Quebec, the question has been posed in early April whether children might be able to return to schools earlier as an attempt to bolster herd immunity. Quebec’s public health director, Horacio Arruda, said in a press conference that it was "very excessively rare” for children to become severely ill as a result of contracting the virus.

But many health experts say plans to use children as an experimental front line for the buildup of immunity brings up ethical questions and we still don’t know enough about COVID-19.

“We need to remember that this is a new virus that we have not known anything about prior to December as far as we know,” Amy Greer, an infectious disease epidemiologist, told CTV’s Your Morning on Monday.

“As scientists, we evaluate the evidence that’s available to us at a given time. Over the course of the outbreak, the evidence will change. But right now, one of the challenges especially is related to children. There’s not a lot of great evidence that tells us what role kids play in the transmission of COVID-19. That’s a real concern.”

Alison Thompson, a professor at University of Toronto's faculty of pharmacy, said letting children be exposed in schools to COVID-19 is a far cry from a mandatory vaccination program.

"It's not just a matter of building immunity -- they have to get sick first," she said in a recent interview, calling the proposal a "sickness strategy."

With files from The Associated Press and The Canadian Press