The seasonal flu tends to dissipate during the summer, leading some to hope the coronavirus will do the same. Experts explain why transmission of some illnesses lowers with warmer temperatures – and warn against lowering our guard.
Why are some viruses seasonal?
Dr Marc Lipsitch: What makes seasonal viruses seasonal is a combination of opportunities for transmission – whether school is in term, which facilitates transmission – and what proportion of the population is immune, combined with weather.
Humidity is lower in the winter, which is good for transmission. Low humidity makes [virus-carrying] droplets settle more slowly because they shrink to smaller sizes and then friction keeps them in the air, whereas high humidity doesn’t do that.
Dr Lee W Riley: People still get the common cold [in the summer] and we’re beginning to see this new coronavirus in the southern hemisphere. It’s more about the way people behave.
Can we expect the number of Covid-19 cases to fall this summer?
Lipsitch: Based on our best estimates from other coronaviruses, summer alone is not going to bring transmission to a level where the number of cases shrinks. It’s just going to grow more slowly.
It’s really clear that warmer weather does not stop the transmission or growth of the virus. That’s clear from Australia, Singapore and Hong Kong. Singapore and Hong Kong have kept it to a large degree under control, but that’s with incredibly intense control measures. There’s no question that coronaviruses are capable of transmitting in hotter, humid climates.
Dr George Rutheford: Thinking that it’s magically going to go away in April or May or whenever is just that – magical thinking. The projections show quite a bit of transmission out through the summer.
Riley: It’s a completely new virus, so it’s really hard to know what would happen. If you try to extrapolate from [related] viruses, then we don’t expect for this new coronavirus to completely disappear by the summer.