The Covid wave fueled by the omicron BA.5 surge is finally starting to ebb in the UK and in some of the harder-hit parts of the US. But why? It’s no longer tenable to argue that disease waves peak and fall primarily because people start taking precautions. People, especially in these two countries, are taking fewer precautions all the time.
Scientists are starting to get a handle on the complex factors that drive waves up and down. Behavior patterns are just one small factor. Changing seasons, new contact patterns and waning immunity can drive waves up, and growing immunity can drive them back down.
For policy makers and scientists, a better understanding of the factors driving pandemic waves would take some of the guesswork out of forecasting and decision-making, such as the commitment to create a booster aimed at BA.5 this fall. Will BA.5 be replaced by then? Or will it resurge in a second wave? Being able to answer these kinds of questions might finally allow politicians and public health officials to get the pandemic under control.
University of Vermont network theorist Laurent Hebert-Dufresne compares each wave to a wildfire burning itself out when it runs out of fuel. Because most people who are infected retain immunity for a few weeks and some for a few months, the disease can — temporarily — run out of people to infect.
And the threshold needed to curb a wave might be lower than we thought. In 2020, the conventional wisdom was that only masks, social distancing and isolation could drive cases down unless herd immunity was reached after the vast majority of people got sick. But a few disease modelers, such as Gabriela Gomes of the University of Strathclyde in Scotland and Tom Britton of the University of Stockholm, predicted that cases might collapse much sooner — after less than a third of the population was infected — because there was a lot of heterogeneity in susceptibility to infection. It was seen as heresy at the time, but turned out to be at least partly right.
In a recent Substack post, physician Eric Topol of Scripps Research uses the term “immunity wall” to collectively describe the various factors that might cause pandemic waves to fall. It’s more complicated than how many people have been infected, vaccinated or both. The bricks in a population’s immunity wall also include which variant previously circulated, when people got vaccinated, the population’s age and overall health, as well as other factors.
And that’s at the national level. Due to variation in the human immune system, some people get longer-lasting protection from their vaccines or previous infections. Waning immunity is a real problem, but it’s far from uniform or universal.
What classic disease modeling can say is that waves that rise fast tend to collapse quickly, said Vermont’s Hebert-Dufresne. That happened with the first omicron wave — the sharpness of the infection curve in the winter of 2021 was surprising, but typical for a very contagious virus. The original US wave in the spring of 2020 was the unusual one because it changed people’s behavior so much.
Bringing back the fire analogy, restricting the amount of contact people had during the initial 2020 wave brought cases down — slowly — but left lots of susceptible people who were then infected during bigger fall and winter waves.
Seasonality adds another layer of complexity. Humidity might make the virus harder to spread in the summer, so winter can bring new waves with dryer air, kids going back to school, and millions of people attending parties and traveling around the winter holidays. It’s clear that the emergence of a new variant such as delta or BA.5 can lead to a big summer wave, though perhaps those waves would have been even bigger had they hit during the winter.
It’s also vital to remember that the amount of testing we do also affects the shape of Covid waves — especially the lack of available tests early in the pandemic and lack of interest in getting tested in this latest wave, said Hebert-Dufresne. That’s another layer of complexity making it hard to know what’s going on now, and hard to predict what will come next. And there are also elements of chance that figure into where outbreaks occur — even air currents in a room can influence who gets infected and who doesn’t.
This can make the rise and fall of infection waves seem kind of arbitrary, said Sam Scarpino, vice president for pathogen surveillance at the Rockefeller Institute and external professor at the Santa Fe Institute. “It isn’t arbitrary, but there are so many moving pieces coupled with shifts in testing and disease severity.”
The BA.5 wave has been the hardest to predict because the lack of testing means we don’t really know what’s going on. Scarpino said wastewater studies have indicated a viral circulation as rampant as last winter’s enormous BA.1 omicron wave — but with far fewer deaths and hospitalizations.
In surveys, people say they’re going back to normal — not necessarily because they think the pandemic is over, but because they value aspects of their normal lives and recognize that the virus isn’t going away. That’s even more reason for science to keep gathering data through wastewater or surveillance testing at universities. Scientists need that data to get a sharper picture of why waves rise and fall — and maybe, eventually, to prevent the next wave from rising at all.