New Brunswick

Herd immunity still an elusive but important goal, virologist says

Herd immunity is still the best goal to slow the spread of the coronavirus, but exactly what that is remains a bit of a mystery, says a professor of virology and immunology.

The more the virus spreads, the more it will mutate

Rod Russell is a professor of virology at Memorial University. (CBC)

Life won't get back to normal until we've achieved herd immunity, says a professor of virology and immunology at Memorial University in St. John's. 

But exactly what that is remains a bit of a mystery, said Rod Russell.

Public health officials have been suggesting "herd immunity" is achieved when 75 per cent of the population is vaccinated against the coronavirus.

Russell said that number is "a guess." 

"There's really no magic number," he said. "We're hoping that 75 per cent vaccination will give us enough herd immunity — that it'll contain outbreaks."

But that still leaves 25 per cent who are still susceptible to the virus, and those people can continue to spread it. 

"To really eliminate the virus from a population, you need to be even higher than that. You need to be up in the 90s."

Whatever the right percentage is for herd immunity, "that's a number that will keep us out of the risk of outbreaks," he said. 

At this point in the COVID-19 pandemic, Russell said, it's more about containment than complete elimination. It's also about lessening the severity of the illness, and making sure hospitals don't become overwhelmed and communities don't have to lock down. 

He said each vaccinated person has "a slightly different range of an immune response."

Russell said our immune systems weaken as we get older. 

"So it's possible that even if you fully vaccinate a 75-year-old person, they might still make a relatively weak immune response, but it should be enough to keep them out of the hospital."

With spread comes mutations

Russell said that as long as the novel coronavirus continues to spread, it will continue to change. 

"All RNA viruses will mutate — and they all mutate all the time," he explained. 

They mutate as a way to get around the body's immune system and continue to spread. 

"That's how they survive — by intentionally making mistakes in their sequences, so that they can have a repertoire of sequences, [so] if an antibody starts to kill one sequence, then you've got another sequence that isn't affected." 

It's survival of the fittest, he said. 

 "As soon as there's a change — if there's a slight advantage to that sequence or that variant — it becomes dominant very fast."

The anatomy of a mutation

Russell said there are 30,000 building blocks in each virus, so there are 30,000 possible changes that can occur. 

He said the same mutation can also occur more than once. For example, the B117 variant that was first identified in the United Kingdom could have also mutated elsewhere in the world.  

"So just because it was labelled the U.K. variant doesn't mean what we see here in Newfoundland came from the U.K. It might just be that the same mutation happened here in St. John's."

He said it's more likely that the variant was brought to Canada by someone travelling from the U.K. and then continued to spread, but it is possible for similar mutations to happen in different parts of the world, unrelated to each other. 

Will vaccines continue to work?

As we've already seen, some mutations allow the virus to spread more easily, which has been fuelling third-wave outbreaks all over Canada. 

The other danger is that new variants of the coronavirus may thwart vaccination efforts. 

Russell said existing vaccines "should be" effective against them. He said everything he's seen so far indicates that the vaccine is effective against the variant first identified in the U.K. 

"We haven't seen any real impact on vaccine effectiveness with that variant. And that one really just has a mutation that makes it spread faster, but it doesn't seem to affect the immune response or the antibody response against it. 

"Now, the Brazilian one, that's a different story," he said, adding he's seen numbers that suggest 10 times the antibodies strength is needed to neutralize the virus with that variant. 

Rod Russell, centre, and his team at the Russell Lab. (Memorial University)

He said there are two mutations that change the shape of the spike protein in an area he called a "nice hotspot for antibodies to bind,"  which makes it difficult for them to do so.   

"There's no doubt … any variant that has either one of those mutations is definitely going to affect how well the vaccines work against it," said Russell. 

Vaccinations will slow the spread of the virus, meaning it won't have as many opportunities to mutate and become even more dangerous. 

"So I've been telling people, if you want life to be somewhat normal, then you need to convince everyone to get vaccinated as fast as possible, because that's the only way we're going to keep from having outbreaks," he said. 

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