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The Omicron variant has made it easier for vaccinated individuals to catch and spread COVID-19, but experts say it’s clear people who have received two or three doses are significantly less likely to transmit the virus compared with people who are unvaccinated.

The issue of transmission among vaccinated individuals has become increasingly heated in recent weeks, as some officials, such as Saskatchewan Premier Scott Moe, falsely claim vaccination is not enough to stop virus spread.

As more provinces move to relax health restrictions and eliminate vaccine requirements for recreation facilities, restaurants and other indoor spaces, Canadians who have been vaccinated could find themselves sharing more space with people who have yet to receive a first dose. Research shows that the risk of COVID-19 tends to increase when unvaccinated individuals infected with the virus spend time indoors with others.

Given that unvaccinated people are more likely to get infected and spread COVID-19 compared with people who have received two or three doses, “there certainly are risks associated with relaxing the vaccine passports,” Jeff Kwong, a scientist with Public Health Ontario and senior scientist at ICES, a research institute based at Toronto’s Sunnybrook Health Sciences Centre, said in an e-mail.

Omicron has only been circulating for a short time, which means scientists are still trying to understand aspects of how and why it is more transmissible. But research indicates vaccinated people are less likely to transmit the virus than the unvaccinated for a few reasons, including being infectious for a shorter period of time owing to a faster immune-system response.

Studying virus transmissibility is complex, particularly during an urgent public-health crisis such as the pandemic. For that reason, Dr. Kwong said, scientists often look to rates of household transmission because the variables are easier to control and it’s much easier to understand virus spread in a household than in a restaurant, mall or other public place.

Researchers in Denmark published a preprint (a study that has not yet been published or peer-reviewed) in late December that showed how likely infected individuals are to transmit the virus to members of their household. The study found the secondary attack rate for the Omicron variant, or the likelihood of a household becoming infected after one individual brings the virus home, is 31 per cent. When the Delta variant was the dominant virus in circulation, the secondary attack rate was 21 per cent. The research found unvaccinated individuals were more likely to transmit the virus. People who had received a booster were less likely to, even compared with people who had received two doses of a COVID-19 vaccine.

Experts say that unvaccinated individuals are more likely to transmit Omicron because they have a higher overall viral load when infected compared with people who have received two or three doses. Meanwhile, vaccinated individuals who become infected with Omicron are more likely to clear the virus quicker than those who have not been vaccinated.

Danish researchers published an updated study last month, after a subvariant of Omicron, called BA.2, began circulating. That study found that the rate of infection in households rose to 39 per cent with the new subvariant, compared with 29 per cent in households with the original Omicron strain (known as BA.1). The research shows the rate of transmission is higher in unvaccinated people; those who received two or three doses were less likely to transmit the virus.

Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said that like everything else in the pandemic, it will take time to have definitive answers about virus spread. But anyone wanting proof that vaccines are slowing transmission should look at the rapidly falling number of cases and hospitalizations, he added.

While Omicron has spread quickly, it hasn’t infected anywhere near enough people to account for the decline in cases we’re seeing to be through natural immunity alone, Dr. Hanage said.

Dr. Kwong said it’s important for people to keep their expectations about the benefits of vaccines in check. While vaccines may not be able to stop all COVID-19 transmission, they work remarkably well at preventing severe illness and death, and that’s what needs to be emphasized, he said.

“The main job of the vaccine is to prevent severe illness,” Dr. Kwong said. “If you’re vaccinated, you’re less likely to transmit it to your family members.”

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