Omicron BA.5 wave is starting to ebb

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. Behaviour 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 get 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 behaviour so much.
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 travelling around the winter holidays.

—Bloomberg