Toronto’s top doctor says that residents who have one dose of the COVID-19 vaccine must not let their guards down amid emerging research suggesting that people can still end up infected, even weeks after rolling up their sleeves.

Medical Officer of Health Dr. Eilleen de Villa made the comment during a briefing on Monday as she revealed that the percentage of residents in some hot spot communities who have received at least one dose of vaccine has now gone from 15 per cent a month ago to about 45 per cent today, in part due to a city strategy to accelerate the rollout in some at-risk postal codes that previously had lower rates of vaccination.

“Now that we've crossed the 40 per cent mark (citywide) and as access to booking expands it is important to remember that the first dose is not full protection,” de Villa warned. “The vaccines work by providing instructions to your body to send an immune response. This includes making antibodies, so that your body is prepared and ready to identify the threat and overcome it. Your cells form a memory of how to do this. The second dose is a vital boost to what the body learned from the first, though. It reminds the cells what they learned and they mount a response again. From that point you are better ready to fight COVID-19 If the virus tries to infect you.”

The first and second doses of the Pfizer, Moderna and AstraZeneca vaccines were all initially administered three to four weeks apart in keeping with the recommendation from their manufacturers.

But the Ford government, along with several other provinces, opted to delay the administration of a second dose by four months in order to get more shots into arms.

This was following research published in the New England Journal of Medicine which used U.S. data to suggest that the Pfizer-BioNTech vaccine could be 92 per cent effective against COVID-19 two weeks following the first dose. Various other studies have provided a range of other estimates for the efficacy of a single dose.

Speaking with reporters, de Villa said that it is uncommon for people to contract COVID-19 after receiving their first shot but she said that it “isn’t impossible.”

In fact, on Monday Health Canada provided data to CP24 revealing that there have been 6,789 confirmed cases of COVID-19 in individuals who had previously received one doses of vaccine so far. About two-thirds of those cases involved people who had been recently vaccinated (within the previous 14 days) while about one-third involved people who had receive their shot prior to that.

Outcome information was only available for some of the cases but there were 203 known instances of someone who had received the vaccine ending up in hospital with COVID-19. Another 53 cases resulted in death.

“We can look at places like the United Kingdom and rightly expect in the near future to enjoy the same kind of progress that they're making but in the meantime while COVID-19 rates are very high in Toronto it remains important to limit your contact with other people you don't live with, to be outside with them, not inside if you have to connect with them and to wear your mask, especially when you can't keep six feet apart,” she said. “Good distance and good ventilation and wearing the mask are what you need to do most right now, whether you're a newly vaccinated, or waiting for your turn.”

About 1.2 million doses of COVID-19 vaccine have been administered in Toronto so far.