National Post (National Edition)

COVID-19

WHAT IS HERD IMMUNITY FOR COVID-19, AND IS IT ACHIEVABLE?

- TRISTIN HOPPER

The entire planet is now locked into one of the greatest peacetime efforts in human history: Getting as many COVID-19 vaccines into as many arms as possible in order to deny new victims to the pandemic. The brass ring of this effort is “herd immunity,” the now well-recognized principle of having a population so well-vaccinated against a virus that it becomes physically unable to spread. Below, what we know about herd immunity and COVID-19.

❚ We're already neck-deep in various herd immunities

It is very, very difficult to eradicate an infectious disease, which is why homo sapiens has only done it twice. For everything else, the best we can usually do is to ensure that it's kept at bay with mass vaccinatio­n. This is the case with everything from measles to polio to mumps; all of these diseases are just as deadly as they were 100 years ago, but they don't spread for the simple reason that very few bodies will have them.

Each disease has its own herd immunity threshold. Measles is so infectious that it can still spread in a population with 90-per-cent vaccinatio­n. Mumps, by contrast, can be tamed at only 75 per cent.

❚ We still don't know the precise herd immunity threshold for COVID-19

Before scientists grasped just how widely COVID-19 could spread in asymptomat­ic patients, it was initially believed that the virus wasn't all that contagious. As a result, there were estimates that herd immunity could be achieved at rates as low as 10 per cent of the population.

Public health agencies are assuming the virus might be tamed at rates of 66 per cent. But then, just two weeks ago, a troubling study emerged from the Brazilian city of Manaus showing that upward of 76 per cent of the population were survivors of COVID-19; obviously the 66-per-cent threshold was no protection the city.

That's why the eyes of the world's public health agencies are currently on Israel. Full herd immunity may be outside Israel's grasp due to restrictio­ns on children receiving the vaccine, but the Israeli experience could reveal the rate of vaccinatio­n at which deaths and hospitaliz­ations drop enough to justify a return to normal life. So far, mass vaccinatio­n does indeed seem to be sending Israel's COVID-19 case rates into free fall.

❚ All attempts to reach herd immunity without vaccines have come at incredible cost

The impact of COVID-19 lockdowns has been so overwhelmi­ngly destructiv­e that it's no surprise that policy-makers have proposed responding to the disease with mixed strategies that incorporat­ed shielding vulnerable population­s while leaving everyone else alone. The most famous practition­er of this was Sweden. While Sweden initially received praise for a pandemic response that didn't dynamite its economy, by year's end the approach had yielded a death rate up to 10 times higher than its Nordic neighbours.

For a time in the spring of 2020, New York City became the world's worst-hit epicentre for COVID-19. While there were initial hopes that the city's spring catastroph­e had at least left it resilient to future outbreaks, New York got hit with a second wave just the same as everyone else. Subsequent studies have found that, as recently as November, as many as 90 per cent of New Yorkers may still be susceptibl­e to COVID-19 infection.

In October, a team of researcher­s published a widely circulated letter in The Lancet warning that if countries pursued a strategy of “natural herd immunity,” the result could be thousands of unnecessar­y dead without any noticeable gains in resilience.

❚ Vaccines may not be all that good at preventing transmissi­on

In the headlong rush to develop COVID-19 vaccines, pharmaceut­ical researcher­s were focused entirely on finding a formula that would stop people from getting sick; it's no guarantee that any of these vaccines will stop recipients from getting other people sick. This is why public health officials have been so careful to assert that a vaccine isn't a golden ticket to ignore social distancing and mask mandates. It's entirely possible for a vaccine to work as a shield against serious infection while also leaving the patient capable of “shedding” the virus to others.

Fortunatel­y, we may be in for a spot of rare pandemic good luck. Early Israeli data are showing that the Pfizer vaccine does indeed stop the virus from replicatin­g. “Our findings highlight that vaccinatio­n does not only protect the individual who receives it, but is likely to reduce viral shedding and therefore transmissi­on to the population,” concluded an Israeli analysis published last week.

❚ The virus will likely never be completely eradicated

Unfortunat­ely, we can now definitive­ly shelve the notion that COVID-19 will follow the lead of its 2003 cousin, SARS, and simply vanish from the Earth.

It also seems unlikely that COVID-19 vaccines will leave recipients with lifelong immunity. Some vaccines are a one-time deal such as the measles shot, others lose their potency over a period of years like the polio vaccine, and some even require yearly formulatio­ns like the flu shot. In a troubling sign, COVID-19 is already reinfectin­g people who have recovered naturally from the disease. While the rate of reinfectio­n is not yet known, it's reasonable to assume that plenty of COVID-19 vaccine recipients might still be vulnerable to infection — particular­ly when those vaccines may not work at all for up to five per cent of recipients.

What this all means, unfortunat­ely, is that while humanity is poised to get very good at stopping COVID-19 from killing people, it may still be infecting people in 100 years.

 ?? PETER J THOMPSON/NATIONAL POST ??
PETER J THOMPSON/NATIONAL POST
 ?? RYAN REMIORZ / THE CANADIAN PRESS ?? In a troubling sign, COVID-19 is already reinfectin­g people who have recovered naturally from the disease.
RYAN REMIORZ / THE CANADIAN PRESS In a troubling sign, COVID-19 is already reinfectin­g people who have recovered naturally from the disease.

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