Immune but infectious: Can someone vaccinated against COVID-19 still spread the virus?

Click to play video: 'Can you be immune to COVID-19, but still infectious?'
Can you be immune to COVID-19, but still infectious?
Health Matters January 6: COVID-19 vaccines have been shown to be highly effective at preventing disease, but can they stop the spread of the virus? As Su-Ling Goh reports, experts say someone who gets the shot can still be infectious. – Jan 6, 2021

As the cross-Canada roll-out of COVID-19 vaccines continues this week, it’s still unclear whether the injections can actually prevent the spread of the virus.

While both the Pfizer-BioNTech and Moderna products have been shown to be about 95 per cent effective at preventing symptomatic COVID-19 illness, there is not much evidence they can protect those around the person who got the shots.

El Sahly was one of the lead investigators for Moderna’s late-stage COVID-19 vaccine trial. She says the novel coronavirus can live in the nasal passage for weeks, meaning a vaccinated person could still infect others, even if they don’t get sick. But there was one promising result in the study.

“We did find, in the short term, that those who got the vaccine were less likely to carry [the virus], but the numbers were really small,” El Sahly said.
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Both the Moderna and the Pfizer vaccines require people to get two doses, about a month apart, to be effective.

Twenty-nine days after their first dose of Moderna, 14 study participants were found to be carrying the virus — versus the 38 people who received a placebo.

“It’s a signal in the right direction, but nonetheless it cannot be interpreted that the vaccine prevents transmission.”

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Alberta’s COVID-19 vaccine rollout rate and the plan to ramp it up

Dr. Jason Kindrachuk, Canada Research Chair in Emerging Viruses, explains the mRNA-based vaccine teaches our immune system to fight the virus, but it doesn’t block it from entering our body.

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“People may be able to still get infected even though they’re vaccinated, but it’s a sub-clinical infection — so they don’t feel sick, they don’t have any symptoms, but they may still be able to transmit,” said Kindrachuk.

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Ongoing research will determine if any COVID vaccine can actually prevent transmission. That will require “collecting a lot of nose swabs on a lot of people,” according to El Sahly.

While asymptomatic carriers are less likely to spread the virus than someone who is coughing and sneezing, masks, distancing and hand-washing will still be critical in 2021 until most Canadians can be vaccinated.

“As we build up that immunity in the public, there is lower and lower… ability for the virus to be able to leap from one person to another,” Kindrachuk said.

Some experts suggest at least 70 to 75 per cent of a population will have to be immunized to control the spread of the virus.

That said, the percentage of the Canadian population that needs to be vaccinated in order to reach confidently herd immunity is unknown, according to Canada’s chief public health officer Dr. Theresa Tam.

“We have an assumption that you will probably need 60 to 70 per cent of people to be vaccinated. But we don’t know that for sure … that’s modelling,” Tam told a media conference on December 4.

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Still, the vaccines are providing some comfort for stressed health-care workers. Pediatric emergency physician Samina Ali received her first dose of the Pfizer vaccine on Jan. 2.

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“It was just so much overwhelming relief… it felt like… as a community, as a world, this was a sign that we were on the way to healing,” Dr. Ali said.

How Pfizer’s and Moderna’s mRNA-based COVID-19 vaccines work

A vaccine is generally made up of a weakened or a dead virus, which, once injected, prompts the body to fight off the invader and build immunity.

Both Pfizer and Moderna’s candidates have been manufactured using mRNA-based technology, a relatively new way to make vaccines.

Instead of injecting a deactivated form of the virus, the mRNA vaccine uses a component of the virus DNA called messenger RNA that basically contains the genetic instructions for the human body to make the specific spike protein of the coronavirus.

By doing this, the immune system learns to recognize and respond to that specific protein, meaning it can more quickly mount a response if the virus enters the body. The mRNA, however, does not modify a person’s DNA or genetic makeup.

“When your body actually sees the real virus, then you have the weapons already in place — the antibodies and the cells that know this virus that can recognize it — and can kill it faster,” Dr. Donald Vinh, an infectious disease specialist and a medical microbiologist at the McGill University Health Centertold Global News.

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According to the data from the clinical trials, Pfizer’s vaccine, which is 95 per cent effective, can offer partial protection as early as 12 days after the first dose.

That protection can last for at least two months, according to Vinh. A second dose is then required to achieve the vaccine’s full potential.

The Moderna vaccine, which also requires a second shot, has shown to be 94 per cent effective.

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— With files from Saba Aziz, Global News

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