Will there be another wave of Covid-19? And if so, how big will it be, and will there be more waves after it?

The answer to those questions depend on seasonality, the susceptibility of the population to the disease, the rate at which the coronavirus mutates and how we come out of lockdown.

Colds and influenza are seasonal because those viruses generally survive outside the body for a shorter time in high heat and high humidity than in cold weather and low humidity. People also spend more time indoors in winter, coming into close contact with others with less ventilation, so respiratory infections are far more common in winter, although of course they can sicken people in summer, too.

But in 1918 and 1919, the years of the worlds deadliest pandemic, the seasons seemed to have little impact on the influenza.

That pandemic had a mild first wave which began in February 1918. It struck relatively few places in the United States or around the world, followed by a lethal second wave which began in Switzerland in late July and spread rapidly around the world from September to December 1918, hitting the Northern and Southern Hemispheres simultaneously. (Australia was hit late; its rigid quarantine of arriving ships delayed the pandemics arrival until January 1919, the middle of its summer.) Then a third wave began in February 1919, marking two distinct pandemic waves in the same influenza season, a highly unusual occurrence.

Susceptibility clearly was a more important factor than the seasons, because it turned out that the entire world young and old people on every continent was susceptible to the disease.

Mutation was also an important factor. It probably accounts for the timing of the third wave in 1919: It seems likely that by then, the virus had changed enough that any immunity to the initial virus didnt protect well against its mutated form. This hypothesis is supported by the fact that exposure to the first wave provided up to 89 percent protection against second wave illness (the best vaccine in the last 15 years provided 62 percent protection), but neither first nor second wave exposure protected against that third wave.

What does all this mean now?

Nothing is certain and little is known about Covid-19, but a few things are likely.

First, modelers estimate that the true number of infected persons is up to 20 times the reported number, which still leaves about 95 percent of the population susceptible. If, as in 1918, susceptibility proves more important than seasonal influences, hot weather will not give as much relief as hoped for. By the same token, that would mean the expected seasonal surge when colder weather arrives might not be as large as feared.

Second, Covid-19 mutates much more slowly than influenza, and its key spike protein the part of the virus that attaches to cells seems particularly stable. Amid all of the bad news that this virus has brought, this characteristic of the virus is a silver lining in several ways.

Since the virus does not mutate nearly as fast as influenza, this reduces almost to zero the chance that it will become more virulent, as happened in 1918. Moreover, because the spike protein is a key part of the virus likely to be recognized by the immune system, then mutation will probably not account for a new wave soon. For the same reason, the consensus view of virologists seems to be that those who recover from the illness probably develop immunity lasting a year and possibly longer, and that a vaccine will most likely protect reasonably well against Covid-19.

Third, the incubation period, on average nearly six days, is roughly triple the average incubation period of influenza, and the disease itself takes much longer for people to recover from and stop shedding virus. Therefore even without social distancing it would take months for the outbreak to pass through a community, as opposed to six to 10 weeks for influenza. With social distancing necessary to reduce deaths by keeping hospitals from being overwhelmed it will take even longer. Additionally, the incubation period allows an asymptomatic person more opportunity to spread disease.

But these factors will give the country more time to expand testing and contact tracing, and to isolate and quarantine contacts. All of those are impossible with fast-spreading influenza.

How then do we restart the economy? We cannot simply wait for herd immunity to develop from natural infection. That would take many months and be accompanied by an unacceptable death toll. Nor can we wait a year or more for a vaccine.

Instead, a consensus has formed among public health experts to continue current measures until the epidemic curve bends significantly downward and the stress on health care is alleviated, followed by a phased-in approach guarded by, in effect, a public health army. That army would be fighting a guerrilla war, armed with tests, tracing, isolation and quarantine to search and destroy inevitable flare-ups.

This approach has worked around the world. It will work here. Covid-19 would continue to spread, but the cases would be in manageable numbers. We would see not so much distinct waves as continuous, undulating swells, broken by occasional angry whitecaps.

But if we do not manage our public health response well for instance, allowing a widespread lifting of restrictions too quickly we could generate a storm surge that washes away everything gained so far by so much sacrifice. That seems to be what too many politicians seem willing to risk.

Those politicians should consider this: In 1918, San Antonio was one of the slowest cities to close, yet one of the quickest to reopen. As a consequence, more than half of the citys population got sick, and almost every household had at least one person ill. And Covid-19 is more contagious than influenza.

Its past time we start doing things the right way. We still lack the testing capacity and anything approaching the necessary public health army. Its past time we start building both.

John M. Barry is the author of The Great Influenza: The Story of the Deadliest Pandemic in History and a professor at the Tulane University School of Public Health and Tropical Medicine.

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