A recent rise in Covid-19 cases in many states across America, including several emerging hot spots are still technically be part of the "first wave" of the pandemic, but that doesn't mean a second wave isn't coming.
So what constitutes a second wave, and what would that look like? Plus, is there anything that can be done to stop it?
Here's what you need to know about a potential second wave of Covid-19.
The United States is still in the "first wave" of Covid-19 infections. "You hear people saying, 'We're heading into a second wave,' [but] it's not," Dr. Greg Poland, professor of medicine and infectious diseases at the Mayo Clinic in Rochester, Minnesota, founder and director of Mayo Vaccine Research Group, tells CNBC Make It. "It's all part of this ongoing reservoir of infection in the U.S."
In order for the first wave to be over, the number of positive Covid-19 infections would have to reach low single digits, Dr. Anthony Fauci, White House advisor and director of the National Institute of Allergy and Infectious Diseases, told The Washington Post on June 18. (In New Zealand, for example, there was a 24-day streak of no infections until June 16, when two travelers who tested positive arrived in Auckland.) "When you get there, then you could feel you have a degree of good control so that when you do get a new infection, you can prevent that infection from becoming a resurgence of essentially many, many more."
After that, a "second wave" would occur when the virus returns or when a new strain of the virus develops, Ian Lipkin, a professor of epidemiology and director of the Center for Infection and Immunity at Columbia University, previously told CNBC.com. For example, the influenza virus causes flu season around the same time (late fall with a peak in winter) each year, although the strain and magnitude of the particular virus can differ.
The next wave of Covid-19 is unpredictable, because there's still a lot about the virus that we don't know, Fauci told The Washington Post.
"[The second wave is] going to depend on your capability and your effectiveness when you do get these little blips of infection, which we will invariably get, that you have the systems, the testing, the manpower to do the identification, isolation and contact tracing," he said. "If you do, it is not inevitable that you're going to have a second wave in the fall."
A second wave would be similar to what we experienced in the spring of 2020, but it could be harder to control in the fall, when people are tired of social distancing, Marc Lipsitch, professor of epidemiology at the Harvard T.H. Chan School of Public Health and director of the Center for Communicable Disease Dynamics, told the American Medical Association in May. It's likely that people would have to follow the same prevention measures that worked in the first wave, such as mask-wearing and social distancing.
The fear is that Covid-19 could return as a second wave in the fall (with peaks in November and December) that coincides with influenza season.
A second wave during flu season could potentially overwhelm the healthcare system, "and we have a big problem," Dr. Marybeth Sexton, an assistant professor of medicine at Emory University in Georgia, tells CNBC Make It. In this scenario, there could be "a huge influx of cases" at a time when people are planning to return to school and work, so they would be harder to address, she says.
Whether or not we see a comeback like this "is at least partially influenced by how well we all do at infection control measures, like distancing, mask-wearing and hand-washing," Sexton says. "So, we may be able to affect change on how high that peak might be." In truth, it's likely that we will continue to see peaks and valleys related to how well people are practicing prevention measures, and when we have an effective vaccine or treatment, she says.
The threat of a second wave is very serious, and could cost people their lives and economic well-being, Poland says. "If we undergo large-scale second waves, you might lose your job," he says. "People don't seem to be making those connections."
So, what can you do? For starters, you should continue to stay away from people and practice hand hygiene. But perhaps most important is wearing a mask when you go out in public, even outdoors.
"If everybody just had a mask on we would dramatically cut transmission," Sexton says. (A recent study from Cambridge suggests that if everyone wore a mask all of the time, it could prevent a second wave of the virus from happening in the next 18 months.)
"You simply cannot get infected with this virus unless you breathe it in, or you touch an infected surface and then touch your eyes, nose and mouth," Poland says. "If you prevent breathing it in, and you prevent touching and contaminated surface and then touching your face, you cannot get infected, which is really powerful information."
Beyond maintaining social distance and wearing a mask when you go out of your home, "the only other thing that will change or mitigate the severity of a second wait is when and if vaccines become available," he adds.
"The best thing that we can do right now is prevent infections, stalling this off as long as possible until a vaccine is available," Poland says. "To do otherwise is ludicrous."