Note: Transcripts may contain errors. If you wish to re-use all, or part of, a transcript, please contact CBC for permission. Please check the corresponding audio before quoting in print. Copyright © CBC 2025

The Dose Transcript for February 20, 2025

Host: Dr. Brian Goldman

[Music: Theme]

BRIAN GOLDMAN: I'm Dr. Brian Goldman. Welcome to The Dose. There is a lot of sniffling, coughing and sneezing going around. That's because case counts for influenza are high. Emergency physicians like me are seeing lots more kids and adults being admitted to the hospital with the flu. Some of them are ending up in the ICU. We seem to be reaching peak flu, so this week we are asking: What do I need to know about this year's flu season? Hi, Zain. Welcome back to The Dose.

ZAIN CHAGLA: Brian, always happy to be here.

BRIAN GOLDMAN: How much influenza are you seeing where you practice?

ZAIN CHAGLA: A lot. It's probably the number one thing that came through respiratory season. You know, the other respiratory viruses have settled down, but influenza seems to be ticking along and reaching its peak in the last couple of weeks.

BRIAN GOLDMAN: So it's capturing the attention of Canadians and our many listeners outside of Canada. So that's why we're talking about it this week. But before we begin, can you give us a hi, my name is? Tell us what you do and where you do it.

ZAIN CHAGLA: Hi, my name is Dr. Zain Chagla. I'm an infectious disease physician out of St. Joe's Healthcare in Hamilton, and an associate professor at McMaster University.

BRIAN GOLDMAN: So Zain, where are we seeing high rates of influenza across Canada?

ZAIN CHAGLA: There is national surveillance for influenza. It really does depend on what areas do the testing. So, you know, not everyone gets tested in the community. A lot of it is hospital-based surveillance. But looking at the Canadian map, we see high burdens in Ontario, in Quebec, in Alberta and Saskatchewan, in British Columbia. It doesn't mean that the rest of the provinces aren't seeing their influenza burden, but there may be some differences in testing to explain some of those differences.

BRIAN GOLDMAN: I'm wondering if you can explain the Public Health Agency of Canada's Flu Watch's method of tracking cases. They call it influenza activity. In layman's terms, what are they talking about?

ZAIN CHAGLA: There's a few different ways that the Public Health Agency does surveillance. Some of it is hospital-based data that gets exported or lab-based data that gets exported through provincial networks. But then there is this flu watchers network, which is really unique in the sense that they have a set number of individuals that they keep asking, Hey, do you have acute respiratory symptoms this week or the next week or the next week? And they track that. And usually, again, it doesn't say whether or not it's influenza, COVID-19, RSV or many of the other respiratory viruses that circulate. But triangulating that onto what's circulating gives us a better sense of how much of this is actually in the community at any given time.

BRIAN GOLDMAN: And they also, as part of that influenza activity, they're tracking the number of tests for flu that turn up positive. Because there's lots of people who have what we call an ILI, which is short for an influenza-like illness. Not all of them have the flu. Some of them will have COVID, some will have RSV, respiratory syncytial virus. But they attach great significance to the percentage of those swabs that are positive for influenza. Have I got it right?

ZAIN CHAGLA: Yeah, absolutely. And so that's part of the reporting. So there are surveillance networks of labs that do testing in the community. But also, yes, that percent positivity is a marker. So, you know, what is the chance, if a clinician orders a test, that it's going to be positive for influenza? You know, is a good serial marker to say, what's the burden in our community? When there's not much, then the likelihood is, is that test is going to be negative. The percent positivity is going to be very low. When there's a ton of influenza, then obviously, you know, most of the swabs are going to come back positive. And so that percent positivity will be tracked over time to see when that peak happens and when the burdens are high.

BRIAN GOLDMAN: As we are speaking right now, the latest data we have -- and there will be updated figures in a few days -- as of February the 8th, the percentage of swabs -- nasopharyngeal swabs, those tests up your nose -- for influenza that were positive was 24.2%. How significant is that?

ZAIN CHAGLA: That's, you know, huge, right? And in fact, Ontario's data, just looking up from last Friday, is up to 40% in some areas. And so--

BRIAN GOLDMAN: Wow, 40%!

ZAIN CHAGLA: Yeah. So, I mean, you could just think of that. You know, a clinician is ordering a test. You know, you in the emergency room are seeing someone with an influenza-like illness. You swab them. 1 in 4 tests are coming back positive, or in Ontario, 4 in 10 tests are coming back positive for influenza. Meaning that, a) people are symptomatic, but b) yeah, it's not much else. It's influenza that's the one that's driving it. And we are pretty judicious about ordering tests, so, recognizing that's a good amount. We also see that percent positivity being driven up by things like outbreaks, for example, where if you have a ward, an outbreak, or a long-term care facility, an outbreak. A lot of those tests are going to end up positive. But 40% is a lot, even 25%. And that really does suggest, again, a huge burden in the community when the probability of testing for it, positive for someone with symptoms, is pretty high.

BRIAN GOLDMAN: You know, my understanding of a flu -- and I've been watching, I've been a flu-watcher for decades now -- the peak usually occurs much earlier in the winter. So we're in the latter part of February. Why is it happening now?

ZAIN CHAGLA: We do see different seasons and different seasonal patterns. You know, pre-pandemic, there have been some seasons where that late peak has occurred, where there has been kind of steady growth. Post-pandemic -- so, you know, the two post-pandemic years we have on record for flu, 2022/23 and 23/24 -- it was much different. It was a very early season. By week 1 or 2 of the year, so kind of early January we saw the peak, and then it declined. A little bit slower of a decline in 22/23 and a very rapid decline in 23/24. You know, this year we're seeing that steady rise basically till about now in mid-February. Hopefully a decline coming from that. And we'll see influenza B come by at the end. You know, it's unclear what the factors are. Are we getting close to pre-pandemic? Are there seasonal trends? The one thing I'll note is that we actually had a very lagging influenza season. So in November, where we start expecting to see influenza picking up in the community, we actually didn't see a lot of it. And so perhaps this is just the season that's moving a little bit to the right to capture that point and, you know, the same burden of cases just over a shifted interval.

BRIAN GOLDMAN: Usually, Zain, we see one dominant strain of influenza A. This year, we seem to be seeing two -- H1N1 and H3N2. What's up with that?

ZAIN CHAGLA: There's co-circulation globally of what's being tested. And again, that's not necessarily the perfect measure. We're seeing H1N1, which is probably about two-thirds to 60% of what's coming back, and then H3N2 being the other percentage of what's coming back. Only a small amount of influenza B, which again, we expect in March. You're right, there's usually, you know, a dominant H1N1 or an H3N2, and often our good graces or bad graces during the season are based on what circulates, as, you know, certain populations may be vulnerable to ones or the other. But we are seeing that co-circulation throughout, that it's not limited to one influenza genotype, it's multiple.

BRIAN GOLDMAN: And maybe that helps to explain why we're having such a stupendous peak at the present time. One question that I know people listening to this will want you to answer: Does any of this have anything to do with bird flu, avian flu?

ZAIN CHAGLA: So no, thankfully. So one of the, you know, advances in pandemic preparedness strategies in multiple surveillance laboratories across the country, and you know, in Ontario, where we are right now, where many of the influenza As are getting sequenced. And so there are surveillance networks making sure that these cases are not H5N1. And particularly for some of those critical care cases, we're trying to make sure most of them get sequenced to get to a diagnostic test of exactly what influenza is causing people to become that sick. Similarly, there's some instructions in areas in people that are bird handlers, agricultural, people who have conjunctivitis, that they get genotyped along with just getting to a diagnosis. And so thankfully, in none of the literature other than that one case in British Columbia, have we seen H5N1 in a human population in Canada. We have just seen a lot of H1N1 and a lot of H3N2 as what's being represented as part of this peak.

BRIAN GOLDMAN: What do we know right now about how severe these two particular strains of influenza A are on people who are getting infected?

ZAIN CHAGLA: The critical care numbers are comparable to prior years. So probably seeing what we would call a bad season between the pandemic year of 2009 and our COVID pandemic. So, you know, we had a couple of bad seasons in that interspace. So, you know, seeing similar utilization in those areas, that's still not insignificant. Obviously, hospitals and utilization are never a great thing. But from a larger standpoint, it does seem like this is a bad flu year. Not as bad as a pandemic flu year, but not certainly some of the lighter years we've seen in the last little while.

BRIAN GOLDMAN: When you say comparable, I guess we can say that we're going to see a similar percentage of people infected ending up being hospitalized, and a smaller percentage of them ending up in critical care or in the intensive care unit, not excess numbers.

ZAIN CHAGLA: Exactly. We're not seeing what we saw in 2009, for example, where health-care utilization was out of control. But, you know, 2016, for example, we had a really bad flu year. And I think we're kind of getting to that trajectory where it wasn't as bad as a pandemic year, but it was certainly a stressful year nonetheless.

BRIAN GOLDMAN: I know you can't say for sure at this point -- we won't know until a little bit later in the year -- but how effective do public health officials and flu watchers believe this year's flu vaccine is?

ZAIN CHAGLA: So a couple of early data points. The Southern Hemisphere data, in terms of prevention of hospitalization -- so again, they look at Southern Hemisphere hospitals, they see who's hospitalized with influenza, and they see who's been vaccinated, who hasn't been vaccinated -- was about 35% protection. Again, seems low, but that's really a vulnerable population where you see that benefit. So, there's that. And then British Columbia does a kind of surveillance-based dataset through Canada. They released their early measures in the end of January and said about a 50% protection against influenza, which is kind of in the benchmark of what we expect. You know, 40-60% is considered a reasonable year. That's very early data based on, I think, October to December surveillance. It hasn't really captured this point in time. And so we do have to wait till the end of the season to see how it all fares out and what the benefits of the vaccine were during this period of surge.

BRIAN GOLDMAN: The production cycle for the annual flu shot is a lot longer than people think. And it's based, as you've said, on the strains of influenza A and B that are contained in the vaccine, or protection against contained in the vaccine, are based on what we know from the Southern Hemisphere. You know, we're talking about a vaccine that was created many months before the rollout in October and November of last year. Have you ever heard of a situation or has there ever been a situation in which a different flu shot was reconfigured in mid-season?

ZAIN CHAGLA: No. I mean, again, most of the technology we use for influenza vaccines is passage through [unclear] and time, so it does take a long time for that production cycle for that vaccine to come through. It would be almost impossible to note that something had happened, be able to reproduce it and have a supply that's reasonable given the millions of doses that are needed. But it doesn't mean that it can't happen. And so, you know, for example, Moderna is developing an mRNA construct for influenza vaccines that kind of builds on the same model with multiple strains -- you know, quadrivalent or trivalent. And so, you know, this might be something in the future where our ability to catch up and compensate might be a tool in our toolkit for immunization, as compared to the traditional symptom of generating vaccines in large vats that take a long time to actually come to production.

BRIAN GOLDMAN: So you could actually make those in a hurry. Those flu shots you can make in a hurry.

ZAIN CHAGLA: Yeah, I mean, the production cycle of an mRNA vaccine is like three to six months. So theoretically, yeah, you could use Southern Hemisphere data to to inform Northern Hemisphere data.

BRIAN GOLDMAN: Of course, the symptoms of the flu -- aches and pains, cough, sore throat, dry cough, often headache, fever of course, and chills. And then there of course are vulnerable populations, people who are young infants, seniors. Especially if they've got what we call medical co-morbidities -- chronic obstructive pulmonary disease, heart failure, kidney disease, etc. And the flu, of course, is not a cold. You know, what are, what are some of the major differences between the flu and a cold?

ZAIN CHAGLA: It's funny, over the last few years, there's always been this adage of 'COVID is the flu.' And 'just the flu' is actually not a great way to characterize it. The flu can be devastating for some people, right? And so, it's a multi-system illness. You know, yes, you have the respiratory symptoms, as you mentioned. People can get profound myalgias and fatigue and so feel just super lousy, hard to recover. They can, you know, occasionally get an organ complication, so infecting their heart, infecting other body tissues. And we also see this interaction with bacterial infections and influenza also being more predominant than some of the other respiratory illnesses we see, and so this bacterial super infection with common bacteria in the upper respiratory tract on people with influenza. And unfortunately, that is a consequence that can lead to severe disease, often even in young, healthy people, which unfortunately I know I'm currently seeing, and can be quite devastating for the wrong population.

BRIAN GOLDMAN: What sorts of things are you seeing?

ZAIN CHAGLA: Acute respiratory distress syndrome; bacterial infections in the blood and sepsis; complications of lung involvement; severe pneumonias, often requiring, you know, surgical or other therapeutic management. And again, we see this every year, but it can be quite profound as some of these populations are just young, healthy, and just poor timing for this virus and bacteria to meet.

BRIAN GOLDMAN: So what can people do to protect themselves and their loved ones?

ZAIN CHAGLA: Lots of information over the last few years about common things to help. So, you know, handwashing, wearing a mask in certain settings, particularly if one are ill. Avoiding, you know, large gatherings when people are sick. Avoiding those vulnerable settings when people are sick. Staying home when one is sick. And really effective vaccinations is still important. And we're at the peak now. There's still some time to be vaccinated, because coming down from the peak, there's still a burden. And then for certain populations, the availability of influenza therapeutics, which may also reduce the severity of their illness or the duration of their illness.

BRIAN GOLDMAN: And not by much. But, so we're talking about medications like oseltamivir or Tamiflu.

ZAIN CHAGLA: Yeah, and again, limited scope of who benefits, and those people at the highest risk of tipping over into severe disease -- you know, some of those very vulnerable groups. But again, it's important for folks to talk to their health-care providers, especially if they do fit into some of those groups. Because the drugs are available, and again, they can speed up the cessation of symptoms and, you know, in some cases may actually prevent some of these folks from ending up in health-care institutions.

BRIAN GOLDMAN: And I want to underscore something that you said very quickly: It is not too late to get a flu shot right now and benefit from it.

ZAIN CHAGLA: Absolutely. It takes about two weeks for efficacy to work. We're at the peak maybe now. Again, it's still unstable data. So that means, you know, if you're on the top of the mountain, there's still a lot of, you know, room to go down the mountain before things get better, right? So we're probably going to see this tail off in March. And we also see influenza B sneak up at the end of March usually. And so, you know, again, there's time to have that protection against influenza B, which is going to give us a smaller but second peak to all of this.

BRIAN GOLDMAN: How will we know that the current outbreak is peaking?

ZAIN CHAGLA: The measures we talked about. Percent positivity, as it starts going down, it means the probability that we're finding people with influenza goes down. Outbreak data across the country usually shifts. You know, it's usually a reflection of community activity. Outbreaks usually start in the community and make it into long-term care facilities and hospitals, and that usually shifts. And then, you know, lingering is the health-care utilization data. We do know... you know, someone gets influenza, they're often not hospitalized on day one. It takes a couple of weeks for them to end in hospital. So, you know, that's probably the more lagging marker of all of this. But the same parameters we look at, as long as they start declining and we never have influenza in the summer, hopefully, again, we'll see that decline coming very soon.

BRIAN GOLDMAN: Could we expect another outbreak from influenza B?

ZAIN CHAGLA: We always see influenza B kind of tailing in that late March period. It never is a huge burden. It does give some burden of illness. It's kind of that second hump. But I think it's expected, obviously. We are seeing just small amounts of influenza B across the country, and so it's, it's ready to rise. But even in years past, in severe years, it's always been influenza A that leads to the big, big stretch in health care, and B being the smaller brother that leads to that second surge.

BRIAN GOLDMAN: When might we see a universal flu shot that protects for years and years so that you don't need the annual flu shot?

ZAIN CHAGLA: So this is a huge biomedical research area that is ongoing globally. The issue with influenza is that the virus continuously shifts. And our best targets, those proteins on the outside of the virus, continuously shift year to year. There's lots of work done trying to identify pieces of the virus, forms of the virus that are more stable over time, and link those to better immune responses so that we actually develop that universal immunity to influenza. Some animal models, work here even at McMaster that's looked at that that has been validated. But to take that from the laboratory and preclinical models to phase 3 trials in the general population is going to take time. So lots of work being done, lots of really interesting technology. The use of mRNA technology also may be a new tool that we can actually make production cycles that are short and adapt vaccines to models that are new. But unfortunately, we're not there yet, and I don't think we can expect one on the horizon anytime soon.

BRIAN GOLDMAN: If I can get political for just a moment, Zain, looking at what's going on in the United States with Elon Musk and withdrawing of funds from institutions that might be important in flu surveillance, how concerned are you and your infectious disease colleagues that the United States might not be a reliable purveyor of information on influenza, and other emerging infections, for that matter?

ZAIN CHAGLA: Surveillance is a global need, and we want to make sure that we have global inputs. The United States is a huge biomedical manufacturer and a huge source of biomedical research. I will say that influenza remains a global threat. And I think even in whatever administration, people know that there are impacts of influenza globally. I mean, again, you know, there are more that died of influenza outbreaks than world wars. And so, I don't think that's a lesson that's lost easily. I think as well, the specter of avian influenza, you know, it has impacts. And, you know, even in the United States now there is challenges with egg supply to the general population because of the effects of influenza. And so it's hard to ignore this virus that is circulating, and I think the impacts that are even happening today, really give a lot of hope that it's one that will not be forgotten in the context of everything.

BRIAN GOLDMAN: Well, Dr. Zain Chagla, I want to thank you for helping to explain in easily understandable terms what's happening with influenza right now. And I hope that you are right and that we are near the peak so that we'll be on the, the happy side of that mountain, as you've described it. And I want to thank you for coming on The Dose to talk about the flu.

ZAIN CHAGLA: No problem. Thanks for having me.

BRIAN GOLDMAN: Dr. Zain Chagla is an infectious diseases specialist at St. Joseph's Healthcare in Hamilton, Ontario. He's also an associate professor at McMaster University. Here's your dose of smart advice. Canada is dealing with a big wave of influenza at the present time. Cases of flu are rising in many parts of the country. Provinces with the greatest flu activity include Quebec, B.C., Alberta and Ontario. Flu season usually peaks in January, but the peak seems to have been delayed, this time until February. In its surveillance of flu, the Public Health Agency of Canada says overall, close to 1 out of every 4 people with flu-like symptoms are testing positive for influenza. One reason why Canada is seeing so much flu at the present time may have to do with the fact that we're dealing with not one, but two strains of influenza A: H3N2 and H1N1. Emergency departments in many parts of Canada are seeing increased numbers of kids and seniors with the flu. Hospitalizations are on the rise, as are admissions to intensive care units. Influenza spreads from person to person through coughing, sneezing or having face-to-face contact. You can also catch the flu when you touch infected droplets from a cough or sneeze, and then touch your own eyes, mouth or nose before washing your hands. The typical symptoms of flu include fever, chills, cough, sore throat, runny nose, muscle or body aches, headaches, fatigue or tiredness, malaise, and to a lesser extent, vomiting and diarrhea. Young children, seniors, and those with heart, lung or kidney disease, diabetes or cancer, tend to be more severely ill and are more likely to require hospitalization. If you get the flu, stay home from work, school, day care, and avoid places where large numbers of people gather indoors. Drink lots of fluids and treat fever and aches and pains with acetaminophen. Anti-influenza medications such as oseltamivir can shorten the duration of illness somewhat, but need to be started as soon as possible after symptoms begin. You can reduce your risk of getting the flu and spreading it to others by getting a seasonal flu shot. It's not too late to get one, though it does take your immune system up to two weeks for the shot to work. During peak flu season, I recommend wearing a mask when attending large indoor gatherings. If you have topics you'd like discussed or questions answered, our email address is thedose@cbc.ca. If you liked this episode, please give us a rating and review wherever you get your podcasts. This edition of The Dose was produced by Sameer Chaabra. Our senior producer is Colleen Ross. The Dose wants you to be better informed about your health. If you're looking for medical advice, see your health-care provider. I'm Dr. Brian Goldman. Until your next dose.

Back To Top »

Provide Transcript Feedback

CBC would like to acknowledge the support of the Broadcasting Accessibility Fund.

W​hat do I need to know about this year's flu season? - Transcript

More Stories Like This

What do I need to know about this year's flu season? - Transcript