It’s been five years since the COVID-19 pandemic began—and a lot has changed.

From early in the pandemic until now, COVID went from causing fear, death, shortages, and uncertainty to being regarded by many people as nothing more than a bad cold.

“We opened up the book to COVID five years ago, and the pages were blank. The pages are now filled,” said William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine.

Here’s what we know—and still don’t know—about COVID.

What We Know About COVID-19

It Can Spread a Few Different Ways

At first, there was uncertainty over how the virus spread. Many people wiped down their groceries and used hand sanitizer to avoid getting the virus from contaminated surfaces.

Data has shown that SARS-CoV-2, the virus that causes COVID-19, is most commonly spread through respiratory droplets and aerosols.

“This is clearly a respiratory virus, and it’s extraordinarily infectious,” Thomas Russo, MD, professor and chief of infectious disease at the University at Buffalo, told Verywell.

Certain People Are More at Risk Than Others

Early in the pandemic, everyone was at risk for developing severe disease from COVID, because they had never been exposed to the virus before, Russo said. While everyone is technically still at risk, data shows that some people are more vulnerable than others.

Those include very young children, older adults, and people with certain health conditions, such as diabetes, asthma, or chronic lung disease.

Vaccines Have Made a Big Impact

The first COVID vaccines were released in December 2020, just months after the pandemic began. “The vaccines are a scientific miracle,” Schaffner said. “They are effective and they are safe.”

Data show that mRNA vaccines are up to 96% effective when matched with the dominant circulating strain of COVID.

“Vaccines against respiratory viruses cannot prevent all mild infections, but they do provide good protection against developing serious illness—they will keep you out of the hospital and prevent you from dying,” Schaffner said.

The Virus—and Symptoms—Continues to Evolve

Today’s COVID is not the same as the original strain. “This is an RNA virus, and these viruses like to mutate,” Schaffner said. “The influenza virus does the same thing.”

The virus has morphed to create several different strains. Omicron variants continue to dominate infections today. Symptoms of the virus have also changed slightly.

“There is now less loss of taste and smell,” Russo said. “The overall symptoms and severity of disease has lessened to a degree,” he added. There are also fewer lower respiratory tract symptoms, such as shortness of breath and chest pain.

For people who have been vaccinated or who previously had COVID, the virus may cause “more cold-like symptoms,” Russo said.

Immunity Wanes Over Time

Most people now have some degree of immunity to COVID, either from vaccination or infection, Schaffner said. The data on how long that immunity lasts varies, but some research suggests that it can last six months.

What We Still Don’t Know About COVID

The Virus’ Origin

This is one of the bigger mysteries surrounding COVID. “There is constant debate about the origins—where did it come from? How did it start?” Schaffner said.

There are multiple theories, including that the virus leaked from a lab or from animals that were handled or butchered by humans. However, scientists believe the virus originally came from an animal, likely a bat.

“Everyone wants to know about the origins, but I’m pretty certain that we’ll never figure that one out,” Russo said.

Why Some People Develop Long COVID

Research has not yet been able to pinpoint why some people develop long COVID and others don’t. “The research into how the virus produces long COVID and how we can treat it is still very much ongoing,” Schaffner said.

Whether COVID Will Become Seasonal

Currently, COVID cases seem to experience peaks in the winter and summer, but they’re not always consistent. Unlike influenza, COVID cases continue to simmer all year. “It’s currently a year-round virus with peaks,” Russo said.