Toronto

Your 1st dose of COVID-19 vaccine is effective but not foolproof, experts warn

Even with one dose of a COVID-19 vaccine, some people are still getting infected and developing severe symptoms that land them in the hospital. Experts are warning people still have to follow public health guidelines, even after that first jab.

It's still possible to get severely ill in the weeks before full immunity develops

Kenny Huynh, 24, gets his first dose of the Moderna COVID-19 vaccine from Dr. Rachel Spitzer at a temporary clinic for residents of Toronto’s northeast Jane and Finch neighbourhood, at the San Romanoway apartments, on Apr. 20, 2021. (Evan Mitsui/CBC)

Health-care experts are warning that even if you've got your first dose of a COVID-19 vaccine, you should continue to observe public health measures or risk getting infected and ending up in hospital with severe illness.

They say although the initial dose significantly cuts the risk of infection and hospitalization, a small number of people are still getting infected and developing serious symptoms that have put them in intensive care units.

That could be because they're coming in contact with the novel coronavirus before the "bare minimum" two weeks the vaccine takes to kick in, said Omar Khan, a biomedical engineering professor at the University of Toronto. 

"That means your immune system didn't have enough time to build up protective immunity to stop severe disease from appearing," Khan said. 

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In the weeks after the first dose, we start developing antibodies to fight against COVID-19, Khan said.

After about a month, our immune system has refined those antibodies, providing more protection against infection and severe illness. Eventually, our immune system develops a kind of memory, so if it comes into contact with the virus in the future, it starts to produce the antibodies again. 

Recent clinical trials back up this timeline. The efficacy of the Pfizer vaccine, for example, is 68.5 per cent one week after participants receive their first doses, according to a study in the New England Journal of Medicine published April 22.

After two weeks, its efficacy increases to 92.6 per cent. Other research suggests Moderna follows a similar trend

Another study found that in Scotland, a single dose of AstraZeneca drove down hospital admissions by 68 per cent one week in. A month later its effectiveness at reducing hospitalizations increased to 88 per cent. 

However, new variants of concern also pose an additional risk. The antibodies people develop from the current vaccines aren't quite as efficient at stopping infection from the mutated versions of the original novel coronavirus strain, Khan said. 

If someone has an underlying medical condition, they're also more likely to develop severe COVID-19-related illness, he said. 

That's why even after receiving a first dose, "hand washing, physical distancing and mask wearing remain as important as ever," said Dr. Theresa Tam, Canada's chief public health officer, on Friday.

The second vaccine dose will remind our immune system how to respond to COVID-19, solidifying that protection. 

Canadians can expect to get their second dose about four months after their first, a longer interval than in some other countries, like the U.S, said Khan. However, it was done strategically to get enough of the population immunized to drive down case numbers and hospitalizations.

"We extended the dosing interval between vaccines to get as many first doses into as many arms as possible, recognizing that the first dose maintains a strong and sustained level of protection from COVID-19," said Ontario Ministry of Health spokesperson Christian Hasse. 

"We have also been clear that people must continue to follow the public health advice after receiving their first dose."

With files from Alison Chiasson

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