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Chief Public Health Officer Theresa Tam appears via video conference as Prime Minister Justin Trudeau attends a news conference in Ottawa on May 4, 2021.Adrian Wyld/The Canadian Press

When Chief Public Health Officer Theresa Tam was asked whether an individual should take the AstraZeneca vaccine offered to them or wait for a Pfizer dose, she talked about evolving science and risk-benefit frameworks, and said more advice will be available before individuals are given second doses.

The amazing thing was, that was the clarification.

Now we have various experts updating the public with information about the risks and benefits of vaccines in a way that provides all possible assistance short of actual help.

You can blame that on Prime Minister Justin Trudeau – and I will get around to doing that – but the truth is the Prime Minister and his aides were pulling their hair out this week. They can’t muzzle the scientists, the experts and the myriad members of advisory panels and public-health bodies, not without a real uproar. But the scientists were making a mess of things.

The message Mr. Trudeau’s government has for months been trying to get out is that the best vaccine to take is the first one you can get.

So when experts on the National Advisory Committee on Immunization said this week that some people at low risk of catching COVID-19 might want to pass on the AstraZeneca vaccine and wait for the “preferred” Pfizer or Moderna jabs, it muddied the message. It raised the perception of risk.

Now, to be fair, NACI had previously called the Pfizer and Moderna vaccines “preferred” because AstraZeneca and Johnson & Johnson vaccines carry a rare risk of serious blood clots. The risk is quite remote, about one in 100,000, many times less than the risk of getting blood clots from birth control pills.

The way NACI chair Caroline Quach-Thanh put it this week – that she couldn’t forgive herself if her sister died from side effects of taking an AstraZeneca vaccine – sure made that risk sound powerful.

She told a TV interviewer people might want to wait if they are in a low-risk area or have low exposure. “What we’re saying is really, do an individual risk assessment,” she said.

Great. Maybe for some places in Atlantic Canada, on some days, you can judge your COVID-19 risk to be low. There was a suggestion that if you work at home and don’t go out, you might want to wait, too. But then you have to calculate your risk from a trip to the grocery store. Is it less than 1 in 100,000, or more?

One thing that science tells us clearly is that people – even smart people, and scientifically trained people – are bad at judging probabilities, and risks. It can be useful if public-health officials give us guideposts, such as telling us we’re more likely to get hit by a car than get a blood clot from the AstraZeneca vaccine, but it’s not much good telling us to guess our personal risk. It’s vaccine advice we cannot use.

So now we have science advisers undermining the public-health message: that the vaccine you are offered will be safe.

Mr. Trudeau’s advisers spent a few hours grinding their teeth about it: NACI had confused people, and might make people more reticent to take vaccines. But this is a PM who’s been telling us to trust the science. He can’t muzzle an advisory panel. How is he going to tell the country this scientist’s message is wrong?

Mr. Trudeau was trying to clean up the mess. He reasserted that all the vaccines offered have been approved by Health Canada. He noted that he and his wife, Sophie, had received AstraZeneca jabs, and that vaccines are the way to end the pandemic. Then he turned it over to Dr. Tam, who was willing to offer a medical opinion as long as it didn’t clarify things.

Dr. Tam said people should be reassured that if they are offered a vaccine, the risks have been weighed. But neither she nor her deputy, Howard Njoo, would answer this straight question: If one is offered an AstraZeneca vaccine now, should they take it, or wait?

That’s where the thing became a farce. You can’t muzzle an advisory panel if it goes off-key. But it’s still Mr. Trudeau’s job to ensure the government responds with clarity, and useful public-health advice to the country. There is a health department, a public-health agency, officials, scientists and Dr. Tam. They all work for Mr. Trudeau. And none of them showed up Tuesday with clear advice.

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