As cases of Covid-19, begin to peak in some parts of the United States, health experts say a variety of factors—including the potential to develop immunity, whether the virus' transmission becomes seasonal, adherence to social distancing measures, and more—will determine whether America faces a second wave of Covid-19 cases and how strong that wave could be.
What could cause another wave of Covid-19?
According to Peter Marks, director of FDA's Center for Biologics Evaluation and Research, until a vaccine for Covid-19, the disease caused by the new coronavirus, is developed, "it's unfortunately not unlikely that [America] may see a second wave or even a third wave" of the disease.
But the severity of those waves will be determined by a number of different factors, public health experts say.
Chief among those factors is whether people who already have been infected with new coronavirus develop any immunity to becoming infected in the future and, if they do, for how long.
Currently, there's not enough data on the new coronavirus to determine whether infected individuals develop immunity, USA Today reports. However, given the virus' similarity to severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS)—diseases also caused by coronaviruses—experts expect that people infected with the new coronavirus will develop at least some immunity against future infection.
Even if infected patients don't develop lifelong immunity to the new coronavirus, the virus eventually might infect enough of the country's population that herd immunity emerges, according to Marc Lipsitch, professor of epidemiology at Harvard University's T.H. Chan School of Public Health. Lipsitch said America could achieve "herd immunity through natural immunity" before a vaccine is developed.
However, Lipsitch cautioned that "there are some indications coming out at the moment that not every case of Covid-19 infection … generates a robust immune response, which would mean that the build up of herd immunity is slower than it's anticipated."
Further, experts say until widespread testing for the new coronavirus is available in the United States, it will be impossible to know how much of the population is immune to the virus. In addition, people see their immunity to some types of coronaviruses wane within a year, meaning America could see additional waves of Covid-19 cases when immunity declines if there isn't yet a vaccine for the new coronavirus.
Experts also note that areas least affected by the new coronavirus now could be breeding grounds for future waves, as there likely will be more people in those areas who don't develop immunity when compared with areas currently seeing large numbers of cases. Gregory Poland—a professor of medicine at the Mayo Clinic in Rochester, Minnesota, and editor-in-chief of the journal Vaccine—said, "This outbreak has predominantly been on the two coasts. Wave 2 will be in the interior of the county where there are a lot of susceptible people."
It's not yet clear how the new coronavirus fares in warmer weather compared with cooler weather, according to Michael Mina, a professor of epidemiology at the Center for Communicable Disease Dynamics at Harvard. "We have no idea really whether this is going to bounce back the moment people start going back outdoors or if the warmer weather is actually going to help us out," he said.
Researchers have noted that many viruses have seasonal fluctuations, such as those that cause the flu, which typically surges during the winter, and Chickenpox, which usually peaks in the spring. According to researchers, those viruses seem to thrive and transmit more easily during colder months and months with more mild temperatures, respectively. While early research suggests the new coronavirus has been less widespread in countries with warm weather and high humidity, researchers and public health experts have cautioned that it's not likely Covid-19 transmission will come to a halt this summer in America. And, even if it does, experts say the country likely would see resurgence in colder months.
"If it doesn't happen in the summer, we'd all be very surprised if we don't see some reemergence in the fall," Mina said.
Whether people continue to practice social distancing and take precautions such as wearing masks in public also could determine whether America sees another wave of Covid-19 cases, experts say.
The Lancet's Shunqing Xu and Yuanyuan Li write that a study on the effects of relaxed social distancing measures in Wuhan, China—where the new coronavirus first emerged—suggested that, if physical distancing measures in the northern hemisphere were gradually relaxed starting in March, the region could see a second wave of Covid-19 cases in the middle of the summer.
Experts say the United States essentially will become a giant testing area as states throughout the country begin to ease social distancing requirements and reopen businesses and schools, allowing epidemiologists to see which measures and precautions are necessary for curbing the virus' spread.
"There are many things we can do, and we don't know how any of them will work yet," Lipsitch said.
How to curb additional waves
Experts say the key to curbing additional waves of the new coronavirus will be increasing testing for the virus and continuing to implement and follow social distancing measures when necessary.
According to Mike Reid, a professor of infectious disease at the University of California-San Francisco, widespread testing and contact tracing is needed to determine who has been exposed to the virus, and those individuals must be isolated for 14 days. "Given the potential for repeated waves, the more that can be put in place to rapidly jump on every new spike and ring-fence every infected individual with wraparound health services is going to be crucial," Reid said.
Barry Bloom, a professor of public health at Harvard, said temperature testing at schools and businesses, also could help. "It's not the most sensitive test in the world, but it is a screen that keeps people conscious that they have an obligation not to infect other people," he said.
Overall, vigilance will be most important, experts say.
William Hanage, a professor of epidemiology at Harvard, said, "No one can say when the coast is clear," and given how easily the new coronavirus spreads and its long incubation period, with just "[o]ne slip … we could see it resurging" (Weise, USA Today, 4/19; Xu/Li, The Lancet, 4/8; Branswell, STAT News, 4/14).