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Dr. E. Kwok administers a COVID-19 vaccine to a recipient at a vaccination clinic run by Vancouver Coastal Health, in Richmond, B.C., April 10, 2021.

JONATHAN HAYWARD/The Canadian Press

More than 8 million Canadians have received a single dose of a COVID-19 vaccine, according to Globe and Mail data. But as the number of partially vaccinated Canadians grows, infectious disease and immunology experts caution against dropping one’s guard against the coronavirus.

Even though a first dose may offer partial protection against death and severe illness, those who have not received their second doses should continue to behave as though they were unvaccinated, experts say. That means continuing to avoid crowds and closed spaces, to wear masks and to practise physical distancing.

“I would argue – and I suspect many of my colleagues who are immunologists and infectious disease specialists who are familiar with the immune response would also suggest – that until you have your second dose, you continue to follow all the guidelines for someone who is unvaccinated,” said Eleanor Fish, professor of immunology at the University of Toronto and emerita scientist at the University Health Network.

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In the U.S., the Centers for Disease Control and Prevention (CDC) have released guidelines for what people can do and shouldn’t do after they’ve been fully vaccinated. For example, they can have in-home visits with others without wearing masks and travel without the need to quarantine afterward. But they’re advised they shouldn’t attend medium or large gatherings, or stop wearing masks and maintaining physical distance when in public and when visiting unvaccinated people who are at high risk of severe COVID-19.

But in Canada, approximately 2 per cent of the population has been fully vaccinated. And as Canadians may wait up to four months before their second doses, which is among the longest gaps between doses in the world, many are caught in an in-between stage, neither fully protected nor fully unprotected against COVID-19.

According to the National Advisory Committee on Immunization, the expert panel advising the federal government on vaccines, all Canadians, regardless of whether they’ve been vaccinated, should continue to practise the recommended public-health measures for preventing the spread of COVID-19.

While Health Canada has authorized four different COVID-19 vaccines – Moderna; Pfizer-BioNTech; AstraZeneca (and Covishield, a version manufactured by the Serum Institute of India); and Janssen, a subsidiary of Johnson & Johnson – Canada has not yet received shipments of the latter one, which requires a single dose. Currently, all vaccines available to Canadians require two doses, and the AstraZeneca vaccine is not recommended for adults under the age of 55.

When it comes to the Moderna and Pfizer-BioNTech vaccines, partial protection only kicks in about two weeks after the first dose is administered, Dr. Fish explained.

Depending on one’s age and health status, “that partial protection varies quite significantly,” she said, which means people are still susceptible to becoming infected, and it’s unclear how sick they could get. Moreover, by extending the time between doses beyond the three to four weeks recommended by vaccine companies, it’s unknown how long that partial protection may last, she said.

“Whether it lasts up to four months or it doesn’t, we have no idea,” she said.

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In addition, the antibodies we generate, called neutralizing antibodies, which prevent the virus from infecting cells by blocking its ability to attach to them, don’t appear to work as well against certain new variants of the virus after a single dose, added Tania Watts, a professor of immunology at the University of Toronto. Two doses, however, are shown to provide good protection, she said.

“That’s one concern, that you’re still vulnerable to these variants and they’re increasing now very quickly in Canada,” Dr. Watts said.

She offered two reasons to continue to adhere to public-health measures after a single dose: “One is to make sure you don’t spread it [the virus] to anyone else because you might not be completely protected. And the other is that you don’t even want to get a mild case of COVID because we’re hearing that even people with mild COVID can have long-term symptoms.”

This is particularly important in the present circumstances in which case counts are climbing and people have a much higher likelihood of encountering someone who is infectious with COVID-19, said Leighanne Parkes, an infectious disease specialist and microbiologist at the Jewish General Hospital in Montreal.

But just because it shouldn’t change your behaviour doesn’t mean you should put off getting vaccinated, experts say. With a single dose, you’re probably still less likely to die or end up in intensive care from COVID-19, Dr. Watts said.

“Having one dose is better than nothing,” she said.

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Eventually, after many Canadians receive their second doses, Dr. Parkes said she expects recommendations, similar to the CDC’s in the U.S., may be given by public-health officials in Canada, which would gradually ease up on restrictions for those fully vaccinated. That could mean, for example, allowing small groups to hold outdoor barbecues without the need for masks, she said.

“The light is there; we just need to be very cautious until we hit that end of the tunnel,” Dr. Parkes said. “We really need to all work together, get vaccinated and keep those public-health measures in place until we’re all fully vaccinated.”

Editor’s note: This story has been updated to include more current numbers on the doses of vaccines given.

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