Is Odd Late Flu Season Par for Post-Pandemic Course?

What does Colorado’s flu season usually look like?

Cases typically start around November and go through March. We usually see cases starting to fall off around April. But this year, we have been seeing influenza cases well into June, which is just something that we don’t normally see. The other thing that we usually see is influenza A circulating first, and then in the second half of the season, we’ll see influenza B activity. But we haven’t really had any influenza B activity, so that’s also really unusual.

How has the pandemic influenced the flu’s more common behavior?

The patterns of influenza have to do a lot with other viruses that are circulating. There is a concept called viral interference where one virus tends to dominate over a given period of time and keeps other viruses at bay. We think that viral interference from SARS-CoV-2 could be one theory as to why we saw an unusual season. When we had the early Omicron wave, there wasn’t a lot of influenza activity. Once Omicron went down, we saw influenza numbers really kick up. Now, after we had the next rise of the Omicron variant, we’re starting to see a steep decline in influenza cases.

What is the southern hemisphere’s flu patterns telling us about what to expect for our fall/winter flu season?

What we are seeing over there in their winter is a very nasty influenza season. They also had a slightly earlier flu season, and that is something we were really interested to know: Are we going to be having an early season that’s almost going to be colliding with this late season? And, again, it has been a more severe season for them, so that does not bode that well for us.

What other infectious diseases are doctors across the country seeing that appear to be higher than usual?

We have seen surges of all sorts of different viruses at unusual times. We have seen a lot of enterovirus and rhinovirus cases. Many children are getting admitted to the hospital with RSV bronchiolitis, something that we typically see more in the winter months. There has been an increase of adenovirus cases, both the kind that cause respiratory illness as well as the kind that can cause gastrointestinal and liver issues. There was recently an outbreak of unexplained liver disease in kids, and a lot of those cases are potentially linked to the adenovirus 41 strain.

What other theories besides viral interference are there when it comes to these changing patterns and new viral outbreaks?

For some of the cases where we are seeing new pathogens emerging that we haven’t encountered before (e.g. monkeypox), there are theories about changing climate and what might have normally remained circulating within animal hosts are now getting into human hosts.

For these unusual patterns of upper-respiratory viruses (e.g., RSV, influenza), one theory is that we did have a lot of human behavior changes at the time of the pandemic, when we were in lockdown and doing a lot of community mitigation. That kept all of the viruses at bay. And now that we have reopened society and we are mingling and are close to all normal activities, we are starting to see a resurgence of these other respiratory viruses.

Many of the infectious disease anomalies are affecting children. Can you talk about that?

Yes, there could be some immunity aspects to it as well, especially for young children. Normally, in the first year or two of life, kids encounter a lot of these pathogens, and then they gain immunity. So the next time they contract a disease, it might not be as severe. There certainly now has been this shift where young children during the pandemic do not have that immune memory. That might translate to more severe disease in a 2- or 3-year-old, versus if they had encountered it when they were younger.

Is there anything else you would like to add?

I think that seeing this unusual flu activity is a good reminder that there are things that we can do to still protect ourselves, including handwashing, staying home when sick and getting the recommended vaccines during the time they are recommended, which can help to prevent the severe outcomes.