Why this flu season is behaving so oddly - An interview with a flu epidemiologist
Why this flu season is behaving so oddly - An interview with a flu epidemiologist
Plus the latest on viral interference, avian influenza, and worrisome vaccine coverage
I interviewed Dr. Matt Biggerstaff, an epidemiologist with the Influenza Division at the Centers for Disease Control and Prevention, for perspectives on why this flu season is behaving so oddly. We also discussed whether “viral interference” could be playing a role and the latest on avian influenza. (I am in bold, Matt is in regular text.)
Why is this flu season starting off so early?
It’s an interesting but complicated question. We have a few hypotheses about why this flu season is starting early. First, we were doing a lot of physical distancing and other interventions during the height of COVID-19. Those measures are relaxing as we move away from the more acute phase of the pandemic and that may be leading to a resurgence in flu activity. The use of masks is decreasing, workplaces and businesses are more fully in person, and those increased contact rates are giving flu a chance to transmit. Second, because we've had a couple of seasons without a significant amount of flu activity, we may now have lower amounts of immunity in the population. And so that may also be contributing some to this early start to the flu season.
It's been my hypothesis as well, that the early season is because of a buildup of the susceptible population. Flu hasn’t spread as widely and so there is a lot more susceptibility or vulnerability to infection at the population level. But I think that some people are hearing this and interpreting it to mean that at the individual level their immune systems didn't get a workout these last few years and so now their immune system is not as good or strong as it used to be.
What I was talking about is on that population level. There was less flu transmitting and so the overall level of immunity has dropped, which might be increasing the chances for flu to transmit. I wasn’t commenting at the individual level. You're not weakening your own individual immune system by not being exposed to the flu. I don't think there's evidence for that. [See also my interview with an immunologist on whether our immune systems were impacted by pandemic precautions.]
What do we know about severity this year?
It's hard to know how the severity of the season is going to turn out. We are seeing really increased flu activity compared to where we are normally, and I think we're at the highest rate of hospitalization for this time in the season compared to the last 10 years. So we have a lot of early activity going on. But so far, we don't know if that's going to translate into an unusually high number of hospitalizations or deaths in the population.
We are seeing signals of decreased vaccine coverage, which is concerning for overall severity [because vaccination helps to prevent severe illness]. We're seeing an especially big decrease in coverage for pregnant women. Right now, coverage is about 12 points lower than where we were at this time in 2021 and about 21 points lower than in October 2020. If those coverage rates don't increase, then we may see worse outcomes at the population level.
We're seeing an especially big decrease in [flu vaccine] coverage for pregnant women.Right now, coverage is about 12 points lower than where we were at this time in 2021 and about 21 points lower than in October 2020.
We're also seeing drops in rates of adult vaccination at pharmacies and doctors’ offices. This year, about 42 million people were vaccinated by the end of October compared with about 46 million by the end of October 2021. So, while we don't know what the severity of this year’s strains might turn out to be, there are some signals that this decreased flu vaccine coverage, especially in more vulnerable populations, is troubling. We want to increase this coverage to at least mitigate some of those severe outcomes.
What about this year's vaccine? Will it be a good match and when will we know?
We won't have flu vaccine effectiveness estimate until later in the season. We need enough flu data to accrue to get stable estimates.
But we do know from the viruses that we've been able to genetically analyze that the match this year looks like it could be good. Most of the viruses that we have analyzed are related to what’s covered by this season's vaccines. We've also done some research using ferrets. Vaccine antibodies produced in ferrets react well against the majority of the flu viruses circulating so far this season. I think those are promising indicators that the vaccine will perform relatively well.
Using Southern Hemisphere flu season data, vaccine effectiveness estimates were around 50% and they had the same dominant strains and vaccine components that we have in the US. So that’s another potential indicator that the vaccine could perform well.
What's the endpoint for that 50% effectiveness? Is it symptomatic illness?
It was about 50% effective against hospitalization.
That's one thing that I found to be a little bit comforting: I had seen that Australia had a very early flu season, but their severity was not in the top tier. So I have my fingers crossed that we will also see moderate severity, but I did not know that our flu vaccine coverage was so low. That's troubling.
Yes, and historically we see that vaccine coverage starts to drop off after the holidays. People stop going to get vaccinated for the flu. We have a short window to get coverage up.
That's an important point. Some people may be waiting to get vaccinated because they are worried about intra-season waning. We know that protection wanes over the course of a season, but how much should we care about that?
We're already in November now, but if people are remembering this for the next flu season, I think it is a balance of how likely you are to return, because getting the flu shot is the most important thing. Waning is not something that should make you delay vaccination if you don't think you're going to go back and get that flu vaccine later.
We do recommend that people, especially groups that are 65 years or older, potentially wait till September or October rather than getting it when it first comes out in August. That timing may be better. But any concerns about waning are definitely not worth forgoing a shot generally.
On the other hand, groups like pregnant women who expect to deliver in August or September should definitely take that flu shot as soon as it's available to make sure some of that benefit is passed on to your baby.
Because you can't get vaccinated until you're six months old, so immunity from the mom is the bridge there, right?
Switching gears, we are having not just a big season of flu, but RSV and Covid are also circulating. Have you given any thought to this idea of viral interference, that the dominance of one virus somehow edges out the other ones?
I think we have some evidence that viral interference has occurred. During the 2009 H1N1 flu pandemic, that displaced a lot of the other respiratory viruses that winter—and there weren't a bunch of community mitigation strategies being deployed in that pandemic that could explain it like there were for COVID-19. But I don't feel like viral interference was the primary driver of those epidemic dynamics.
It's also really complicated to untease. We have non-pharmaceutical interventions [like masks] coming on and off, we have different immunity profiles occurring with flu and RSV not circulating as usual the last two years, and then we have vaccine induced immunity in play for flu and Covid. There's a lot going on, and I don't always know if it's true viral interference, like “Covid prevents your flu infection for the next month.”
And think with Covid, when we had the big Omicron variant surge or other big surges, millions of people were out because they're sick and so they weren’t out and about in the community, able to get infected with other viruses. It's a complicated phenomenon.
It's been a big year for avian flu, too. I was wondering if you could talk a little bit about what's going on with bird flu.
Yeah, we're having a lot of “big years” right now. Flu is enjoying its time back. What we know about the avian or bird flu situation is still mostly an animal health issue. We haven't seen a lot of human activity from it yet, but it's starting to approach a record year. More than 49 million domestic birds in 46 states have either died as a result of the bird flu infection or have been culled because of exposure to this virus. The previous high was in 2015, where the number was 50.5 million, so we're quite close to surpassing that. During the 2015 outbreak, we only had 21 states affected and we're at 46 states so far this year. So we're already seeing a lot more geographic spread. Most of the country has been impacted in one way or another by this outbreak, which wasn't the case in 2015.
While we think the overall risk to the general population is low, we still want people to take preventative measures around any potentially infected birds or poultry just to make sure we prevent the spread of these viruses to humans or other birds. Flu has a propensity to find new hosts and so we want to try to limit the chances for this virus to jump into a new species. It's something we're watching—and unfortunately, we're heading into another fall or winter where activity may pick up.
In addition to not wanting avian flu to get a running start in humans, are we seeing any impacts in consumer products like eggs or turkey dinner?
There is definitely an agricultural impact. There are numerous stories of turkey and other poultry products like eggs going up in price and that is definitely having an impact on the consumer. We don't believe that there are any food safety issues, though. People should not be worried about getting bird flu because of their turkey dinner, by any means, but there are effects on the producers of these products and also on the price that people are paying.