KATELYN JETELINA MPH, PHd

AUG 24, 2023

The fall/winter season always means more respiratory sickness—the weather changes, people head inside, and social networks change (school starts, holiday celebrations occur). But the last four years have resulted in very unusual patterns.

This makes predicting this season a fool’s game. But I’ll take the bait and give it a shot.

What makes predicting challenging?
Three main factors:

Many scientists agree that SARS-CoV-2 still hasn’t settled into a predictable, seasonal pattern. BA.2.86 is an excellent example.

Changing human behavior: The changing immune landscape, new and improved tools, and pandemic fatigue have meant that human behavior has changed drastically from season to season, as I outlined in a rough table below. Human behavior plays into viral patterns.

Perplexing viral behavior: Other viruses’ timing seems to be off. One hypothesis is viral-to-viral interaction—one virus takes over, then subsides, and another virus wave begins due to population-level immunity to the first virus. For example, last year, we were concerned about a triple-demic—all three viruses peaking simultaneously and flooding our hospital systems. Interestingly, they didn’t, which helped immensely. (Viral-to-viral interaction? Maybe—we’ll see what happens this year.)

(Hospitalizations per 100,000 for RSV (green), flu (blue), and COVID-19 (red). Source: CDC
But I think this season will be the first time we have a repeat in behavior (except for the RSV vaccine availability) and all three viruses present.

So, my guess: this year’s patterns will broadly mirror last year’s.

So what happened last year?
RSV arrived earlier than usual (peaking in early November) and with a vengeance, particularly among children under 5. Our pediatric hospitals were overwhelmed, to say the least.

(CDC)
Flu also came earlier than usual, peaking around mid-November. (Pre-pandemic it peaked in mid-January.) But last year’s wave was about the same height as pre-pandemic.

(CDC)
COVID-19 had a middle-of-the-road year. In other words, hospitalizations weren’t as bad as before but still had a sizeable impact.

(CDC)
Of course, BA.2.86 on the scene complicates fall/winter predictions. The COVID-19 Modeling Hub—8 academic teams across the U.S.—made projections before BA.2.86 was identified but took this kind of scenario into account:

If BA.2.86 explodes, we likely see closer to scenario F

If BA.2.86 fizzles, we likely see scenario E

This means we should expect to lose 55,000 (optimistic model) to 450,000 Americans (pessimistic model) over the next two years.

Scenario Modeling Hub projections for COVID-19 deaths across four scenarios
What happened in the Southern Hemisphere?
The Northern Hemisphere has an advantage in predicting upcoming seasons—Southern Hemisphere data. Their respiratory season occurs before ours, so we can look to their winter season to predict ours.

This year’s Southern Hemisphere respiratory season looked a whole lot like last year's:

In South America, RSV made huge waves, just as high as last year.

Respiratory Syncytial virus distribution and percent positivity by subregion, 2015-2023, Region of the Americas
Flu hospitalizations in Australia were as early and about as high as 2022.

Number of influenza hospitalizations at sentinel hospitals, from April to October, 2016 to 2023 by year and week of diagnosis. Source here.
Excess deaths in Australia continue to tick up from pre-pandemic levels.

The all-cause death rate per 100,000 population, all ages, 2017 to 18 June 2023. Source here.
Bottom line
I wouldn’t be surprised if the Northern Hemisphere saw a repeat of last year: an early RSV wave, followed by an early flu wave, and COVID-19 finishing us off. But of course, predicting patterns only 4 years into a novel virus is a fool’s game.

Regardless, we know viruses are coming. Their wave height depends heavily on how we utilize our tools, like vaccines, ventilation, medications, and masks. Let’s consider using them.

Love, YLE

“Your Local Epidemiologist (YLE)” is written by Dr. Katelyn Jetelina, MPH Ph.D.—an epidemiologist, wife, and mom of two little girls. During the day, she is a senior scientific consultant to several organizations. At night she writes this newsletter. Her main goal is to “translate” the ever-evolving public health world so that people will be well-equipped to make evidence-based decisions. This newsletter is free, thanks to the generous support of fellow YLE community members.