Viral forecast: Experts break down what to know about infectious diseases in circulation

Canada has officially lost its measles elimination status. Here’s how to best protect yourself against that highly contagious illness — as well as COVID-19, RSV, influenza and the common cold.

Each autumn, in tandem with the changing leaves and the onset of sweater weather, there is a surge in seasonal illnesses. A host of factors — children returning to school and spending time in close quarters, less ventilation and exposure to dry HVAC-circulated air as temperatures drop, the effects of weather fluctuations on immune systems — lead to more cases of the flu, COVID-19 and the common cold. This fall, in addition to the usual suspects, measles, one of the most contagious infectious diseases, is spreading in Canada.

A number of circumstances — including issues around accessibility, growing hesitancy and vaccine fatigue in the wake of the COVID-19 pandemic — are contributing to a decline in vaccination rates in Canada. Despite recommendations from the National Advisory Committee on Immunization that anyone six months of age and over receive the COVID-19 vaccine, only 3.9 percent of the population did so last year, creating a greater risk to the population this fall and winter.

COVID-19, the flu (influenza), the common cold and respiratory syncytial virus (RSV) are all considered respiratory viruses, characterized by a fever with a cough or sore throat, alongside a list of other possible ailments, including body aches, sneezing, nausea and loss of appetite, according to the Pan American Health Organization. “That’s why we sometimes just call it the respiratory season, because there is quite a mixing pot of viruses,” says Dr. Reed Morrison, a public health physician at Public Health Ontario.

With so many infectious diseases in circulation, what’s the best defence? Here’s the latest medical advice.

What is the fall forecast for flu, RSV and COVID-19?

It can be difficult to predict how acute the season will be, given the many factors at play, including vaccine uptake, virus strains, and whether COVID-19 and RSV have peak activity around the same time. Early forecasts in the U.S., however, indicate that we could have “a similar season to last year, which was a pretty severe year for flu,” says Andrea Thomas, director of epidemiology at BlueDot Global, a Toronto digital health company that provides alerts, data and assessments on the spread of infectious diseases globally. (By Thomas’s account, severity refers to the number of people whose illness results in hospitalization or death.)

Influenza and RSV typically peak in December or January, “and we can probably expect that again this year. And the common cold will circulate throughout fall and winter, says Dr. Morrison. “There is likely to be at least some overlap in activity between these three major viruses as well as COVID-19,” he adds.

To keep track, a weekly survey on rates of flu, RSV and COVID-19 is available at the Public Health Agency of Canada (PHAC) online: Canadian Respiratory Virus Surveillance Report. So far, the rates of infection for flu and RSV have been low. COVID-19, however, circulates year-round and doesn’t follow the same seasonal patterns.

Why is COVID-19 hard to predict?

SARS-CoV-2 (the virus that causes COVID-19) has a tendency to change. Reports suggest the latest subvariants, characterized by severe sore throats and fatigue, are behind the recent rise in infections. BlueDot, which uses AI to collect data from a number of sources in order to predict and track the spread of infectious diseases, hasn’t detected the emergence of any cause for concern, says Thomas. “We do have good global population immunity to the virus in general. So we’re in a much better place than we were a couple of years ago. But it is still an evolving virus,” she adds. “It’s quite possible we’ll see a new divergent variant because this virus is able to mutate.”

Barring the unpredictable, Dr. Morrison expects rates of COVID-19 infections, which rose over the later part of the summer and early fall, to continue to decrease. “We may already have seen that fall peak,” he says. “But there is always a chance that we could see another peak later in the winter.”

“Staying up to date with recommended vaccines is one of the most important ways to protect yourself from developing serious complications from respiratory illnesses such as seasonal influenza, COVID-19 and RSV,” says Anna Maddison, a spokesperson for Health Canada and the Public Health Agency of Canada.

How should we think about herd immunity?

“Herd immunity is when a disease does not spread well in a population, because there’s a high proportion of that population that is immune, which can be either from vaccination or previous infection,” says Dr. Morrison.

When a high enough percentage of a given population has immunity, those who are immunocompromised or who cannot be vaccinated are “protected by the rest of the ‘herd,’” according to the Canadian Institutes of Health Research.

“I get a vaccine not just for myself, but because I know it lowers the risk of infection,” says Thomas. “It lowers the risk that I will bring something home to my family or that I will be spreading one of these diseases to other people.”

How could changing policies in the United States impact the availability of vaccines in Canada?

Canadian health experts have expressed concern that changes to health policies in the U.S. could affect which vaccines are manufactured and at what volume, and could also increase the spread of vaccine skepticism here. These shifts may have potential negative health outcomes for our population.

But while some of Canada’s vaccines are sourced from American companies and facilities, supply of vaccines is not an issue. “Canada has access to sufficient supply of mRNA vaccines to meet the requirements of provinces and territories for the fall and spring campaigns,” says Maddison. Plus, there has been a concerted push to shore up Canada’s biomanufacturing capacity in recent years. To manufacture its Canadian doses, Moderna is using a new facility in Laval, Que., and its syringes will be filled in Cambridge, Ont.

If you got the COVID shot last year, do you need to get vaccinated again?

“COVID vaccines are updated annually to provide the best protection against the strain of virus that we are likely to encounter each year,” says Dr. Morrison.

Health Canada has approved two mRNA COVID-19 vaccines (Moderna’s Spikevax and Pfizer-BioNTech’s Comirnaty) with the updated variant for this year.

The most recently updated COVID-19 vaccine is expected to boost an individual’s existing immune response and to provide “a more targeted immune response against currently circulating COVID-19 strains,” adds Maddison. Plus, a recent study found that the COVID-19 vaccine provides added benefits; research from the MD Anderson Cancer Center in Houston and the University of Florida reported that it boosted the immune system of cancer patients, helping them respond better to cancer treatment.

Flu vaccines, which are also updated each year to target the variants likely to cause the most infections, are available free of charge across Canada. The data on vaccine protection is solid, says Dr. Morrison. “We can avoid getting sick from these viruses,” he adds. “And if we do get sick, vaccines can make it less severe.”

How big of a risk is measles this fall?

Canada successfully eliminated measles in 1998. But after 12 months of a large outbreak in many regions across the country, Canada was notified by the Pan American Health Organization that it has lost its elimination status. Though the number of cases are down, the disease is still spreading in several areas around the country. As of early November, there have been 5,138 cases according to the Canadian Measles and Rubella Surveillance System, a significant jump from 147 cases in 2024.

“We still have not gotten back to the vaccine coverage we had even in 2019. There’s still many pockets of the population that have below-herd immunity,” says Thomas. “I expect to still see this causing spread throughout different areas of Canada until we get our vaccination coverage back up again. It’s just such a contagious virus.”

As the vaccination rate for the first and second doses of measles has declined in Canada, so has the overall immunity, which puts highly susceptible individuals, such as unvaccinated babies, at risk, according to Gavi, the Vaccine Alliance. Earlier in October, a baby in Alberta died after being born prematurely to a mother who contracted measles during pregnancy. Another baby died in southwest Ontario, in June, under similar circumstances.

How do you know if you are protected against measles?

“Immunization is the most effective way to protect ourselves and others against measles,” says Dr. Morrison. “The first thing that someone could do is to check their immunization record.”

The recommended schedule and number of measles vaccine doses varies, depending on when you were born and, for some populations such as healthcare workers and military personnel, what previous immunity testing you may have had. “A healthcare provider will be able to advise on measles immunization based on each person’s specific history,” says Maddison.

“In general, for people who have not been previously infected with measles, one dose of measles-containing vaccine provides up to 95 percent protection against measles, and two doses of vaccination provides almost 100 percent protection,” she adds. “A third dose of the measles vaccine is not recommended, nor is routine testing for laboratory evidence of measles immunity.”

Unlike viruses like COVID and the flu, measles is not similarly prone to mutation. A 2021 study at the Mayo Clinic found that the likelihood of a new measles variant emerging and evading current vaccine immunity was “near zero.”

Are there any implications for getting vaccines for the flu, COVID and measles all in one season?

“Any or all vaccines that a person is eligible for can be administered at the same visit. For example, this could mean getting the COVID-19, influenza and RSV vaccine at the same visit,” says Dr. Morrison. “This is really helpful in a lot of circumstances for most people just to save on time.”

What misinformation or disinformation is causing Canadians the most harm?

Recent studies published by the U.S. National Library of Medicine have demonstrated how social media, and such factors as risk perception, opinions on government response to the pandemic and the endorsement of misinformation have contributed to vaccine hesitancy and the erosion of trust in public health institutions. A recent survey from the Canadian Medical Association showed that 43 per cent of Canadians are highly susceptible to believing misinformation and another 35 per cent are moderately susceptible. More than one-third of survey respondents had skipped effective health treatments as the result of false information.

“The growing role of social media as an information source, artificial intelligence and entities working to spread false information have shaped the information environment and made finding credible, evidence-based information about vaccines more difficult,” says Maddison. This misinformation and disinformation, she adds, can give rise to a sense that one can disregard public health guidance, “which can lead to preventable illness and death.”

What is the best way to address vaccine hesitancy?

In addition to misinformation and disinformation, vaccine hesitancy can be driven by multiple factors, according to the CMA, including, ironically, the fact that vaccines are so effective, which reduces the overall risk posed by those diseases and leads some people to forego the vaccines that created the protection in the first place.

“We know there’s a lot of reasons why people may choose not to be vaccinated,” says Dr. Morrison. “You really have to find out what the barrier is for that individual and address it. Some people have heard that the disease itself is not very severe. In this case, it’s really important for [people working] in public health and healthcare to share why we are concerned about the disease. And that may include talking about some of the complications, or some of the really severe examples.”

In other cases, Dr. Morrison says, people have concerns about whether vaccines are safe. “That’s why we talk so much about how rigorous that process is of approving a vaccine, and how robust the safety monitoring is,” he adds.

For those with questions about the safety or effectiveness of vaccines, Maddison recommends contacting a health-care provider, public health authority or referring to credible sources such as Canada.ca.

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