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 Rutherford: 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.

Could there be a second wave of infections in the autumn?

Riley: A reintroduction of the epidemic is certainly possible; it’s beginning to happen in Hong Kong. Hong Kong successfully controlled the epidemic early on, and they started relaxing some of their restrictions. Now they’re beginning to see new cases reappear.

Rutherford: In 2009, we saw a second wave of the swine flu. It started in the spring in Mexico. Some schools in the United States got out early for the summer, and when [students] went back in the fall, it came back in a bump. That [outbreak] was blunted because a vaccine came out that fall. That’s not going to happen here. We’re going to have to temporize until a vaccine arrives.

Lipsitch: If we let up on social distancing before a large fraction of the population is immune, there could be a second peak of infections in the fall, when it’s most contagious, due to school being back in session and cooler temperatures. And that would be the worst possible outcome.

Will we need to resume social distancing in the fall?

Lipsitch: If our strategy is to use social distancing as our main control measure because we haven’t figured out anything better, then the best way to do it is to distance until we bring cases down to low enough levels that we can let transmission resume by relaxing social distancing, and have several weeks or months where we don’t overwhelm the healthcare system.

And then we distance again, and repeat the cycle. With each cycle, we’ll get more time off social distancing, because the buildup of immunity in the population helps to slow the spread. So you don’t get to the dangerous peak as quickly.

That [scenario] will be destructive to the economy, to education, and all sorts of things. But as a means of trying to preserve the healthcare system, if we don’t have another approach, it may be our best option.

I want to be clear: as an epidemiologist, I’m saying what I think existing tools make possible for the purposes of disease control, and not what I think is socially desirable. Multiple rounds of social distancing are not something I look forward to.

Riley: It’s conceivable that we may have to do another round of lockdowns, but we need to look even further ahead. What’s going to happen next year? Is it going to come back again like the influenza? Is a new type of coronavirus going to come back? Maybe not next year, but maybe, two years from now? This is not the only time we’re going to be doing these lockdowns.

Is there another approach we could take?

Rutherford: I think as shelter in place starts to get peeled back, it’s going to need to be replaced with something more along the South Korean model of aggressive contact tracing, quarantine and isolation, and that’s going to be the bridge to get us out to when the vaccine comes in. Given the hit on the economy that’s going on now, there’s going to be a lot of enthusiasm for the South Korean model.

Lipsitch: If we can do that, it’s great. The challenge is that reintroductions are a constant threat. We’ve seen it in China. They’re trying to go back to work while doing control based on individual cases, but they’ve had multiple introductions from outside the country now. I think it’s what we should aim for, but I’m not hugely optimistic that it will work.

Riley: South Korea and Hong Kong had really efficient contact tracing programs, where they would quarantine the contacts of symptomatic people who were diagnosed with coronavirus. It was a much more focused approach to controlling transmission.

The problem in the US is we don’t have that kind of manpower, and that’s probably something that the US really needs to start looking into in a very serious way, because we just totally neglected our public health system infrastructure.

Panel:

Dr Marc Lipsitch, professor of epidemiology and director, Center for Communicable Disease Dynamics, Harvard TH Chan School of Public Health

Dr Lee W Riley, professor and chair of the Division of Infectious Disease and Vaccinology, UC–Berkeley School of Public Health