A the pandemic stretches into its third year, Covid-19 restrictions are being lifted around the United States. But how will the pandemic officially end? Writing for the Associated Press, Mike Stobbe explains how three previous epidemics could offer some insight.

Influenza

Before the Covid-19 pandemic, influenza was widely regarded as the deadliest pandemic agent. For instance, during the 1918-1919 flu pandemic, there were an estimated 50 million deaths around the world—with 675,000 in the United States. From 1957 to 1958 another flu pandemic killed an estimated 116,000 Americans, and another outbreak in 1968 killed 100,000 more.

Then, in 2009, a new flu brought on another pandemic. However, it was unexpectedly less dangerous to the elderly—the group that is generally most at risk of death from the flu and its complications. Ultimately, there were fewer than 13,000 deaths in the United States that were attributed to the 2009 outbreak.

In August 2010, the World Health Organization (WHO) officially declared the flu had transitioned into a post-pandemic period, with cases and outbreaks following a customary seasonal pattern.

Notably, many experts believe the coronavirus will follow a similar path.

"It becomes normal," said Matthew Ferrari, director of Pennsylvania State University's Center for Infectious Disease Dynamics. "There's a regular, undulating pattern when there's a time of year when there's more cases, a time of year when there's less cases. Something that's going to look a lot like seasonal flu or the common cold."

HIV

In 1981, health officials identified multiple cases of cancerous lesions and pneumonia in gay men who were previously healthy across California and New York. Within a year, officials had classified the disease as acquired immune deficiency syndrome (AIDS).

Then, researchers determined AIDS was caused by human immunodeficiency virus (HIV). For many years, AIDS was considered a death sentence—it even became the leading cause of death for Americans ages 25 to 44 in 1994.

However, treatments that emerged in the 1990s turned AIDS "into a manageable chronic condition for most Americans," Stobbe writes. As a result, efforts shifted to other areas of the world, where it was is still considered an ongoing public health emergency.

Typically, pandemics don't end with a disease subsiding uniformly across the globe, according to Erica Charters of the University of Oxford. "How a pandemic ends is generally by becoming multiple [regional] epidemics," she said.

Zika

In 2015, Brazil experienced an outbreak of infections from Zika virus. It quickly became a "terror" when scientists discovered that infection during pregnancy could result in a birth defect that impacts brain development—which causes babies to be born with abnormally small heads.

By the end of that year, the disease was spreading across other Latin American countries. In 2016, WHO declared Zika an international public health emergency.

Soon after, CDC received reports of 224 cases of Zika transmitted through mosquitoes in the continental United States and over 36,000 cases across U.S. territories—most of which came from Puerto Rico.

However, cases declined dramatically in 2017 and virtually disappeared soon after. In the United States, experts believe the epidemic died as people developed greater immunity to the virus. "It just sort of burned out" and the pressure to create a Zika vaccine available in the United States quickly subsided, said Denise Jamieson, a former CDC official who was a top leader in the agency's Zika responses.

While it is possible the virus will remain dormant for years, outbreaks could still occur if the virus mutates or if a large number of young people lack immunity. Ultimately, with most epidemics, "there's never a hard end," said Jamieson.

What needs to happen before we are 'out of [the Covid-19] pandemic?'

Before WHO declares an end to the international health emergency, enough countries will have to first see a significant decline in Covid-19 cases—or, at least, a notable decline in hospitalizations and deaths, Stobbe writes. However, WHO has not yet announced these target thresholds.

This week, agency officials addressed questions regarding the possible end of the pandemic by citing the progress that still needs to be made "before the world can turn the page," Stobbe writes.

Although Covid-19 cases have declined by 5% globally in the past week, cases are currently rising in some areas, including the United Kingdom, New Zealand, and Hong Kong. In addition, many countries still need vaccines and medications for their citizens, said Carissa Etienne, director of the Pan American Health Organization (PAHO).

"We are not yet out of this pandemic," said Ciro Ugarte, PAHO's director of health emergencies. "We still need to approach this pandemic with a lot of caution." (Stobbe, Associated Press, 3/10)