Separating noise from signals: How ‘adverse reactions’ to COVID-19 vaccines are investigated

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Possible side effects of the AstraZeneca vaccine
WATCH: Possible side effects of the AstraZeneca vaccine – Mar 12, 2021

Amid lingering concerns over blood clots among AstraZeneca COVID-19 vaccine recipients, the pandemic is seeing other potential buzzwords emerge: “adverse reactions.”

Some people are going to experience “adverse events” post-vaccination, experts say. The challenge for scientists though is determining whether the reactions are directly related to a vaccine or merely coincidental.

It’s a key distinction, but experts say it may be misunderstood, leading people to jump to conclusions about vaccine safety that simply isn’t justified.

“Sadly, in this time frame, some people are going to have heart attacks, some people are going to have blood clots. Pick any symptom — someone is going to have it because we’re dealing with tens of millions of people,” said Isaac Bogoch, an infectious disease specialist based out of Toronto General Hospital.

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“It certainly warrants an investigation. But it doesn’t necessarily mean it’s related to the vaccine.”

How does it work?

Different countries have different mechanisms for determining adverse effects after vaccination.

In Canada, reports on COVID-19 vaccine safety are released weekly on an official website. They’re broken down into categories, each encapsulating mild to serious potential reactions.

Non-serious events, like soreness at the injection site, and serious adverse events, like hospitalizations, are all tallied and reported. But the reports are also separated by their link to the vaccine, or lack thereof.

“Just because it’s an adverse event, doesn’t mean it’s directly related to the vaccine. It just means that it happened after someone got a vaccination,” Alyson Kelvin, a virologist at Dalhousie University, said in a previous interview with Global News.

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“I could get the vaccine and five hours later get hit by a car, and that’s going to be recorded as an adverse event.”

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It’s like separating a true connection from a coincidence, said Kelly Grindrod, a pharmacy professor at the University of Waterloo, and there a number of ways scientists analyze this.

Timing and proximity play a big role, she said.

“Let’s say you were destined to have a heart attack tomorrow … If you were given a new drug or vaccine the day before, because of proximity and time, we need to consider if it’s related,” she said.

“But the thing is, you were always due to have that heart attack. We have to figure out which of the people were going to have it happen anyway and by coincidence they got vaccinated and which people weren’t destined to have a heart attack.”

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The same analysis is happening with AstraZeneca, she said, and will with any other reactions after any vaccine.

As it stands, there is no evidence the vaccine was responsible for the blood clot reports in Europe. The drugmaker said Sunday that, out of more than 17 million people vaccinated with the drug, there have been 15 events of deep vein thrombosis and 22 events of pulmonary embolism as of early March. The reports led to a number of countries choosing to suspend the use of the vaccine, despite regulators around the world, including Health Canada and the World Health Organization, saying there is no indication the shot is responsible for the reactions.

The other way researchers determine whether an adverse event is related to a vaccine is finding out how common the same reaction is among the population in general. The U.K., which was the first to widely dole out the shot, has said any reports of blood clots among its citizens were not greater than what would have occurred normally.

“We look at what the baseline rate is in a country to begin with, and try to answer whether it’s happening more often in the groups we’ve given the vaccine,” Grindrod said.

“It’s the signal to noise ratio. These are signals in a ton of noise. Scientists have to figure out what the noise is, get rid of that, and find out if what’s still sitting there is actually still a signal or was it all noise.”

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Noise in numbers

Experts say it’s inevitable that Canada’s number of adverse event reports after vaccination will increase over time.

As more people are vaccinated, more incidents will pop up, but they will be rare and generally mild, Bogoch said.

As of March 5, there have been 1,923 adverse event reports after COVID-19 vaccination — that’s 0.085 per cent of all doses administered to that date. Of those, 1,709 were deemed non-serious and 214 were deemed serious.

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Many of those reports were “vaccination site pain” and “vaccination site redness.” Others reported headaches, hives or itching at the shot site.

The Public Health Agency of Canada only lists about 40 reported adverse reactions following immunization on its website. It also includes rare, but more serious reports — like anaphylaxis, muscle pain and difficulty swallowing. However, there are less than 50 reports of these reactions out of all Canadians vaccinated so far.

Experts stress that adverse reaction reports can really be a “whole range of things.”

In the U.K., for example, drug analysis documents layout reactions reported from recipients of each individual COVID-19 vaccine. In a 63-page document specific to AstraZeneca, there are reports of blood clots forming after vaccination, but also claims of dry lips, “diet failure” and “crying.”

“Any adverse reaction reported — they’re going to collect the data,” said Bogoch.

But, as health-care providers, “we err on the side of caution,” said Grindrod.

“Just to be safe we fill out an adverse event reporting form and we send it off. It’s up to them at the Health Canada level to see if they get a lot of the same thing coming in. That’s what happened in Europe.”

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The adverse events of “special interest,” or “safety signals,” is what deserve particular attention, as these are actually related to the vaccine. So far in Canada, there have been no reported safety signals.

“Meaning that even though (a number) adverse events have been reported, none of them have been directly linked to a life-threatening adverse event following immunization, that’s related to the vaccine,” Kelvin said previously.

“We’re not getting any type of safety signals here in Canada.”

System nothing new

The pandemic may have put a laser focus on side effects related to vaccines, but experts emphasize that system for reporting potential unusual reactions is nothing new.

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The Canadian Adverse Events Following Immunization Surveillance System (CAEFISS) continuously monitors the safety of all marketed vaccines in Canada — not just ones for COVID-19.

Reports of vaccine-related reactions by recipients — no matter how small — are siphoned through local health-care providers and provincial public health authorities, eventually landing in the CAEFISS database, which is run by the Public Health Agency of Canada.

The data is used to identify adverse events that may require more in-depth investigation. The same goes for drugs and other pharmaceuticals, via the MedEffect Canada system.

“We’re not making this up as we go along,” Bogoch said. “These have existed for decades, and COVID-19 is no different.”

Ultimately, the system is working exactly the way it’s supposed to, said Grindrod.

“Most people just don’t pay attention to this kind of health care,” she said.

“This is science in real-time. It’s a very normal process. It’s the difference between drug trials and real life.”

— With files from the Canadian Press

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