COVID vaccines are on the horizon, but how long might protection last?


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The vaccine that pharmaceutical company Pfizer reported on this week may be 90 per cent effective, as their press release suggests, but questions remain about just how long that effectiveness will last.

This is of particular concern given recent reports of waning immune antibodies in people who have recovered from a COVID-19 infection and reports of rare cases in which people have been reinfected after already having had the coronavirus once.

But one of the researchers on the front line of Canada’s COVID-19 vaccine testing said there’s reason to temper that concern and maintain optimism about durability of the protection vaccines may provide. 

“Most of the new vaccines that are in phase three trials [including Pfizer’s vaccine] have shown very robust antibody responses to the vaccines that they’re testing,” said Dr. Joanne Langley, the co-lead of Canada’s COVID-19 Vaccine Task Force, pediatric infectious disease specialist and vaccine researcher at Dalhousie University’s Canadian Centre for Vaccinology.

Our immune system generates antibodies after it recognizes a foreign invader — be it the SARS-CoV-2 virus, if you become exposed, or a vaccine against it. These antibodies specifically target the virus to neutralize it. But a “robust” vaccine response doesn’t mean it will be long lasting.

“We’re trying to train the human immune system to really recognize the [viral spike] protein, so that if it ever sees it again, it will be able to mount an army that’s specifically targeted to that particular pathogen,” Langley told Quirks & Quarks host Bob McDonald. 

The Pfizer vaccine is an RNA-based vaccine that contains the genetic instructions for the SARS-CoV-2 coronavirus spike protein (in red), which then get constructed in our own cells, so that our immune system can recognize that protein and mount an immune response should it encounter it again. (The Canadian Press)

Within the ranks of our immune system’s “army,” the first molecules to face the battle are only able to recognize if a particular invader is foreign or not, so they’ll attack indiscriminately against anything that shouldn’t be there. 

But our immune system’s coordinated intelligence units — which get recruited when we’re exposed to a natural infection or a vaccine —arrive later to gain critical “intel” to recognize and remember the virus, said Langley. This way it can mount a prompt and effective immune response should it encounter the invader again.  

The Pfizer vaccine is made to stimulate a immune reaction to the virus similar to this natural immune response, so the body “remembers” the virus and is prepared to neutralize it before it can cause illness.

But how long that response lasts after encountering the virus — or a vaccine — is still an open question.

Concern around fading antibodies after infection

In a study of more than 100,000 people who’d tested positive, scientists in England found that antibodies against the COVID-19 coronavirus dropped by 26.5 per cent during the three months of the study period.

They charted this decrease in antibodies in all areas of the country and in each age group, except for healthcare workers, suggesting that a higher initial exposure or repeated exposures could help maintain antibody levels. 

And while that study has yet to be peer reviewed, Dr. Langley said not every infection results in antibodies circulating in the blood for a long time.

Perhaps a vaccine, which in many cases, is a more potent immune stimulus than the wild infection, could result in protective immunity and hopefully get us out of this pandemic.– Dr. Joanne Langley, Canadian Centre for Vaccinology

Some antibody responses can be detected for many years after the infection or inoculation. This the case, for example, with the tetanus vaccine, said Langley. But the antibody response to other pathogens, like the hepatitis B virus or vaccine, become undetectable over time.

That’s not necessarily a problem, however, because the immune system can remember the threat and make new antibodies if challenged by it again. A hepatitis booster vaccine, she said, can quickly lead to “rocket high” levels of antibodies.

So while it would be encouraging if antibodies did persist for a long period after a COVID-19 infection, this simply may not be how the immune response to this particular infection behaves. Protection could still be effective and durable. It’s a concern but not a “death knell” for the COVID-19 vaccine or infection-acquired immunity, said Langley.

Immune system impact of prior coronavirus exposure 

The few cases of reinfection have also raised concerns that a vaccine may not be effective against different strains of the virus. One published case found that the second infection was with a slightly different genetic variant. Dr. Langley said this is something she and her colleagues will be watching closely. 

“That’s concerning, but I think it’s important to note how rare it is,” she said.

So far the changes that have been seen with the virus aren’t significant from an immunological perspective for the kind of protection we can expect after an infection or vaccination.

The vaccine would still be expected to work ” even though the virus has changed a little bit over time,” she added.

And some encouraging research has suggested that infection with older, non-COVID coronaviruses, may give rise to partial protection against the pandemic coronavirus. These are viruses that have circulated in humans for many years that typically just cause cold symptoms.

Despite the rising COVID-19 cases in many provinces throughout Canada, including Ontario, anti-mask protesters marched in close proximity to each other without masks through the streets in Aylmer, Ontario on November 7th. (Geoff Robins / AFP via Getty Images)

A recent study published in the journal Science suggests that five per cent of adults — and 43 per cent of children — retain antibodies against previous coronaviruses that can block SARS-CoV-2. 

“That’s very encouraging,” said Langley.

“Perhaps a vaccine, which in many cases is a more potent immune stimulus than the wild infection, could result in protective immunity and hopefully get us out of this pandemic.”

Produced and written by Sonya Buyting



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