Why a Second Wave of Covid-19 Is Already a Worry: QuickTake

As authorities the world over consider when to lift economy-crippling movement restrictions aimed at curbing coronavirus infections, the fear on everyone’s minds can be expressed in two words: second wave. The concern is that, once quelled, the pandemic will resurface with renewed strength, causing a repeat of rising infections, swamped health systems and the necessity of lockdowns.

1. What’s a second wave?

Pandemics are caused by new pathogens that the vast majority of humans have no immune protection against. That’s what allows them to become global outbreaks. Pandemics are uncommon, but influenza is one of the more frequent causes. What often happens is that a novel variant of flu virus spreads around the world and then recedes, kind of like a tsunami. A few months later, it comes back and spreads around the world, or large parts of it, again.

2. What makes the first wave recede?

Influenza pandemics can be temporarily beaten back by the change of seasons, moving to the southern hemisphere when the northern half of the globe heats up during its summer, and vice versa. The virus may also have infected a huge portion of people in most areas, giving them immunity from re-infection and possibly creating so-called herd immunity, which protects those who haven’t been infected by curtailing the virus’s circulation. In the case of the coronavirus that causes Covid-19, countries around the world have adopted movement restrictions on an unprecedented scale and social-distancing measures that together keep people far enough apart so that the virus can’t easily spread.

3. So how does a virus come back?

There are a number of possibilities. In the case of influenza, there’s the onset of cool weather, a factor that may affect the coronavirus, too. Or the pathogen can mutate. This is another feature of flu, which mutates more or less constantly. In the fall of 1918, a second wave of the historic influenza outbreak occurred and caused most of the deaths in the pandemic. Some researchers believe it was brought about by a mutation that made the virus again unrecognizable to most people’s immune systems. Another important variable is the movement of the virus to populations that haven’t been exposed before and don’t have immunity. In addition, the World Health Organization said on April 24 that there’s no evidence yet that people who have recovered from Covid-19 and have antibodies are protected from a second infection.

4. Why wasn’t there a second wave of SARS?

The 2002-2003 outbreak of severe acute respiratory syndrome in Asia never reached the scope of a pandemic. Though caused by a coronavirus, it wasn’t as contagious as the one responsible for Covid-19. Its spread was mainly restricted to hospitals and other settings where people came in close contact with the body fluids of infected patients. Ebola is another pathogen relatively new to humans. There have been periodic outbreaks in Africa, but while the virus is highly contagious in some settings, it hasn’t been sufficiently infectious to spread around the world like the coronavirus.

5. What are the prospects for second waves of coronavirus?

There have been hints from China that a second wave is a risk. Some areas of the country that were shut down by the virus and then reopened had restrictions reimposed in March because of new cases. Much of the rest of the world is still struggling to get the current wave under control. Most areas that have contained the virus have done so using movement restrictions, which slow the virus’s spread but leave many people vulnerable to infection once they begin to venture out again, raising the prospect of second waves.

6. What could prevent them?

The WHO has recommended lifting movement restrictions in stages to test the effect of each before moving to greater openness. In any case, experts say, the key to keeping infections low without locking down everyone is to scale up testing and contact tracing. Health authorities need to find infected people, isolate them, and identify their recent contacts, so they can be tested as well and isolated if necessary. Eventually, it’s possible that enough people will become exposed to the coronavirus that herd immunity will develop and it will stop spreading, or that a vaccine against it will be licensed.

The Reference Shelf

• Related QuickTakes on what you need to know about the pandemic, the role of silent spreaders, how the pandemic will end, the role of testing, the quest for treatments and a vaccine, and the seasonality question.
• Bloomberg News explains how countries are watching China closely for signs of a second wave.
• Subscribe to a daily update on the virus from Bloomberg’s Prognosis team.
• The U.S. Centers for Disease Control and Prevention has a coronavirus web page.

--With assistance from Paul Geitner.

To contact the reporter on this story:
John Lauerman in London at jlauerman@bloomberg.net

To contact the editors responsible for this story:
Eric Pfanner at epfanner1@bloomberg.net

Lisa Beyer, Timothy Annett

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