At the outset of the pandemic, one of the most troubling scenarios discussed by some epidemiologists was the ability for COVID-19 to become endemic, or in other words, a recurring virus that circulates in humans. There are currently four known coronaviruses that regularly infect people, mostly causing common-cold type symptoms but on rare occasions, pneumonia, and even death.

The question was whether this latest new virus would mark the fifth endemic coronavirus, a development that would influence how policymakers wage their battle against COVID-19.

Now, two scientists at Columbia University are renewing the debate just as a second wave threatens to take hold across Europe and in the Northeast United States. In an ominous sign, infection rates in New Jersey and Connecticut recently became high enough for them to qualify for New York's 14-day quarantine criteria for out-of-state travelers.

In a new article published last week in the journal Science, public health researchers Jeffrey Shaman and Marta Galanti detail the precise circumstances in which coronavirus becomes an enduring public health problem.

The paper is not predictive. Instead, Shaman and Galanti identify the key factors in determining the trajectory of COVID-19, namely, the likelihood of reinfection, the effectiveness of a vaccine, the seasonality of the virus, and whether significant numbers of people will contract COVID-19 along with another virus like influenza.

"Should reinfection prove commonplace, and barring a highly effective vaccine delivered to most of the world’s population, SARS-CoV-2 will likely become endemic," the authors conclude.

Although top U.S. health officials, including Dr. Anthony Fauci, have expressed confidence that a vaccine will be approved by the end of the year, some experts have warned that the first round of vaccines may only provide moderate protection at best, short of what is known as "sterilizing immunity," which fully stops the pathogen from infecting a host, and making masks necessary through 2021.

In one seemingly doomsday scenario, the first round of vaccines will be mediocre, and antibodies from initial infections will prove insufficient in warding off reinfections. On top of that, those reinfections will induce illnesses as severe as the first infections did.

How likely is all this to happen?

“The answer is we don’t know," Shaman told Gothamist. "This is not to fear monger. We can’t dismiss that possibility.”

To date, only a handful of people have been identified as having suffered a repeat coronavirus infection, although scientists say that it's impossible to discern how widespread reinfections are since the majority of those who contract the virus again go undetected. Among the concerns is whether reinfected individuals can transmit the disease to others. A Hong Kong patient who was determined in August to have suffered a second bout of coronavirus was also found to have a high viral load despite being asymptomatic.

Writing in the Lancet last week, a group of scientists pushing back on herd immunity invoked the uncertainty surrounding reinfections.

"It is unclear how long protective immunity lasts, and, like other seasonal coronaviruses, SARS-CoV-2 is capable of re-infecting people who have already had the disease, but the frequency of re-infection is unknown," they said.

Even in the advent of an effective vaccination, Governor Andrew Cuomo recently said that he thought “micro clusters” of the virus could be a reality for as long as a year. He cited the time it would take for a vaccine to be distributed as well as the prospect that some people will refuse to get vaccinated. 

Shaman agreed, saying that flare-ups could very well continue into the unforeseen future.

Amesh Adalja, an infectious disease specialist at the Johns Hopkins Center for Health Security, who in February raised the specter of coronavirus becoming endemic said the latest data had only strengthened his convictions.

"The likelihood is even more so," he said. "We’ve seen how efficiently this spreads, how many cases there are."

"This is a disease that by definition is not going to become containable," he added.

He argued that in light of this understanding, governments should be focusing their efforts on developing tools that decrease the severity of coronavirus such as antiviral treatments as well as vaccines.

Stephen Morse, an epidemiology professor at Columbia University’s Mailman School of Public Health, also spoke in February about the possibility of coronavirus becoming endemic.

He said such an outcome still strikes him as "plausible," adding that the process is still a mystery in many ways. Researchers, he said, don't know very much about when and how today's endemic coronaviruses originated and became “domesticated” as human infections.

"We often take the endemic coronaviruses for granted. They’re usually thought to cause very minimal disease, but a recent study suggested that coronaviruses may cause comparable or even more severe disease than flu," he added.

Ultimately, only time will tell if coronavirus will produce reoccurring outbreaks.

"Nobody can do challenge studies on this. You can’t do something in a lab. We have to wait and see," Shaman said.

Denis Nash, an epidemiology professor at CUNY, agreed, saying that scientists only have data on the new disease beginning from January.

"To go through an entire year is important," he said. Unfortunately, he added, "We’re about to get a sobering look of what it's going to look like in the fall in New York City."