Can COVID-19 vaccines reduce transmission? Here’s what we know so far

If you have received the coveted coronavirus vaccine shot, there is both good and bad news.

While there is a high probability you will not get sick with COVID-19, there is a chance you can still catch the novel coronavirus and in turn infect others.

Read more: So you got a COVID-19 shot. Now what can you do?

“Vaccination doesn’t prove that you are immune,” said Alberto Martin, professor of immunology at the University of Toronto.

Canada has so far approved four different coronavirus vaccines. During the global clinical trials, the shots from Pfizer, Moderna, AstraZeneca and Johnson & Johnson showed high efficacy rates – ranging from 62 per cent to 95 per cent – in preventing moderate to severe COVID-19 illness.

But there are still many unanswered questions about whether the vaccines can also suppress transmission – that is, keep vaccinated people from acquiring the virus and spreading it to others.

There are a number of ongoing studies looking at the effectiveness of the vaccines on transmission – and preliminary data is promising.

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According to a pre-print analysis in The Lancet medical journal – not yet peer-reviewed – the AstraZeneca-Oxford vaccine was shown to reduce the rate of positive PCR tests by half when two doses were given. There was a 67 per cent reduction after one dose.

A study by Cambridge University in the U.K. suggests that a single dose of the Pfizer-BioNTech vaccine can reduce fourfold the number of asymptomatic COVID-19 infections. These results released on Feb. 26 are yet to be peer-reviewed.

Read more: How long will the COVID-19 vaccine protect you?

Meanwhile, another Israeli study published in the Lancet medical journal on Feb. 18 found an 85 per cent reduction in symptomatic COVID-19 infections within 15 to 28 days of receiving a first dose of the Pfizer vaccine, with an overall reduction in infections, including asymptomatic cases detected by testing, of 75 per cent.

These are encouraging signs for ending the pandemic, as preventing infection in those who have been vaccinated means there is no virus that can be passed on to others.

If you never get infected, then it’s also possible that you won’t transmit it to other people,” said Gerald Evans, an infectious disease specialist at Queen’s University in Kingston, Ont.

Pfizer is currently testing a subset of Phase 3 trial participants to look for evidence of symptomatic and asymptomatic infection.

“Understanding whether the vaccine can stop the transmission of the virus will be an important tool for the continued rollout of the vaccine and to inform public health planning efforts and recommendations,” Christina Antoniou, director of corporate affairs of Pfizer Canada, told Global News in an emailed statement.

According to the World Health Organization (WHO), there is limited evidence about the performance of the COVID-19 vaccines to prevent infection or transmission.

However, it is “reasonable to assume there will be some level of protection against transmission,” the U.N. health agency said in an emailed statement to Global News.

Evans said while more data is needed to determine the vaccination’s impact in real-world settings, he believes vaccines are the “number one tool” at our disposal and will substantially curb COVID-19 spread.

I’m estimating about 75 to 80 per cent reduction in transmission,” he said.

“That’s the way we’re going to be able to go into a post-pandemic world.”

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Easing of restrictions due to vaccinations not happening soon

Martin is also confident that vaccines will stop the spread of COVID-19 or any other disease, for that matter.

Once you establish immunity to a specific virus, you can no longer transmit it,” he said.

He explained that the immune memory established by the vaccine prevents the virus from producing billions of viral copies needed for a person to become infectious.

“So you yourself are protected and you protect others around you,” he said.

Read more: Can delaying a second COVID-19 shot save lives? Some experts say so

There is also evidence that the viral load, which determines how much virus you can emit when you cough, breathe or sneeze, is significantly lower when you are vaccinated.

Results of a pre-print study done in Israel — not peer-reviewed — suggested that the Pfizer-BioNTech vaccine can reduce the viral load for infections by fourfold 12 to 28 days after the first dose is given.

“These reduced viral loads hint to lower infectiousness, further contributing to vaccine impact on virus spread,” the authors said.

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The Travel Lady: Will vaccination passports play a role in future travel?

Evans said this is important because high viral loads are not just associated with more severe disease but also with a higher risk of transmission.

“If the infection you get is associated with a really low viral load, then that means the likelihood that you’re going to go on to transmit it to somebody else is significantly and dramatically reduced.”

— with files from Global News’ Linda Boyle 

© 2021 Global News, a division of Corus Entertainment Inc.

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