A lot has changed since early 2020, when countries around the world first realized the potential threat of a highly contagious, and still mysterious, flulike virus.

In the early days of the coronavirus pandemic, no one knew for sure how the virus spread. People were scrubbing their groceries. Governments urged people to stay home, to wash their hands frequently and to avoid touching their faces.

Masks quickly emerged as a point of confusion, as public health officials at first discouraged people from wearing them, citing shortages, and then endorsed them. Mask mandates became a flash point in the culture wars as states, counties and cities across the country adopted a patchwork of policies.

On Tuesday, the Centers for Disease Control and Prevention said that it was no longer necessary for fully vaccinated people to wear masks in small groups outdoors, bringing the public guidance in line with a growing body of research indicating that the risk of spreading the coronavirus is much greater indoors.

Here is how the public health guidance on masking in the United States has shifted since the start of the pandemic.

February 2020

‘Stop buying masks,’ surgeon general pleads

“Seriously people — STOP BUYING MASKS!” the surgeon general at the time, Dr. Jerome M. Adams, wrote on Twitter in February 2020. “They are NOT effective in preventing general public from catching #Coronavirus, but if health care providers can’t get them to care for sick patients, it puts them and our communities at risk!”

Dr. Adams said in another post that the best way for people to avoid catching or spreading the coronavirus was by washing their hands often and by staying home if they felt sick.

At the time, masks — particularly N95s, which are thicker, fit more tightly around the mouth and nose, and block smaller particles than surgical masks do — were in high demand, leading to price gouging. Shortages abounded in hospitals across the country.

Even Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, voiced concern at the time that urging Americans to wear masks could lead to even worse shortages of medical masks, including N95s.

“You don’t want to take masks away from the health care providers who are in a real and present danger of getting infected,” Dr. Fauci told CNN.

On March 15, the C.D.C. made no mention of masks when it recommended that gatherings in the United States — including weddings, festivals, parades, concerts, sporting events and conferences — be limited to 50 people.

April 2020

A change in policy, with more mixed messaging

In April, officials reversed course, with the C.D.C. urging all Americans to wear a mask outside their homes to supplement other public health measures, such as social distancing and hand washing.

Masks were recommended for all people over age 2 who were in a public setting, traveling or around others in the same household who might be infected. However, President Donald J. Trump immediately undercut the message by saying it was voluntary and by vowing not to wear a mask himself.

Officials said masks should be worn primarily to reduce the spread of the virus, not necessarily to protect the wearer.

In April, American Airlines, Delta Air Lines and United Airlines joined other carriers in requiring passengers and flight attendants to wear a face covering.

September 2020

Health officials speak out for masks

Many officials have emphasized the public health benefits of masks. In September, Dr. Robert R. Redfield, then the C.D.C.’s director, told a Senate committee that masks were “the most important, powerful public health tool we have” for fighting the pandemic, adding that the universal use of face coverings could bring the pandemic under control in months.

“I might even go so far as to say that this face mask is more guaranteed to protect me against Covid than when I take a Covid vaccine,” Dr. Redfield said. Vaccines, he said, are not 100 percent effective, whereas masks, when worn properly, do what they are designed to do.

However, Mr. Trump quickly rejected those comments, saying Dr. Redfield had “made a mistake” in suggesting that masks may be more useful than a vaccine.

The next month, Mr. Trump again undermined the guidance from Dr. Redfield and other public health officials in his administration when he removed his mask for the cameras as he returned to the White House from the Walter Reed medical center, where he had been hospitalized with Covid-19.

January 2021

President Biden imposes some masking rules

President Biden in January used his executive authority to impose mask requirements where he could — including on federal property and in interstate travel.

In a series of orders, Mr. Biden made mask wearing mandatory in airports and on many airplanes, as well as on intercity buses and on trains. He also urged all Americans to “mask up” for 100 days.

March 2021

The C.D.C. issues its first guidelines for vaccinated people

In March, almost exactly a year since the pandemic first gripped Americans in fear, the C.D.C. said that people who had been fully vaccinated against the coronavirus could gather in small groups indoors without masks or social distancing. Vaccinated adults could begin to plan mask-free dinners with vaccinated friends, the agency said.

March 2021

States begin lifting mask mandates

With vaccinations on the rise, some states began lifting mask mandates. Others, including Florida and South Dakota, never had one.

Gov. Greg Abbott of Texas, a Republican, lifted the mask mandate and capacity limits on all businesses starting March 10. The order ensured that “all businesses and families in Texas have the freedom to determine their own destiny,” Mr. Abbott said.

Utah, Arizona, Iowa and Wisconsin did the same.

The governors of Montana, North Dakota and New Hampshire allowed statewide mask mandates to expire. Gov. Eric Holcomb of Indiana, a Republican, would follow suit in April by replacing a statewide mask mandate with an advisory.

Other states remained strict: In Massachusetts, for instance, outdoor masking was still required at all times, even when nobody else was around.

April 2021

C.D.C. relaxes masking advice for people who gather outdoors

On April 27, the C.D.C. said that fully vaccinated people generally no longer needed to wear masks outdoors, but should continue to wear them at indoor gatherings or at crowded outdoor events. People who haven’t gotten their shots can also go without a mask in small gatherings held outside as long as they are with fully vaccinated friends and family, the agency said.

Vaccinated adults should continue to wear masks and stay at least six feet from others in large public spaces — such as at outdoor performances or sporting events, or in shopping malls and movie theaters — where the vaccination and health status of others would be unknown, the agency said. And they should still avoid medium-size and large gatherings, crowds and poorly ventilated spaces, officials said.

A growing body of research indicates that the risk of spreading the virus is far lower outdoors than indoors. Viral particles quickly disperse outdoors, public health officials have said, so the transmission risk is far lower, though not impossible.

“I think it’s pretty common sense now that outdoor risk is really, really quite low,” Dr. Fauci said Sunday on “This Week” on ABC. Particularly “if you are a vaccinated person, wearing a mask outdoors — I mean, obviously, the risk is minuscule.”