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

In the early days of the coronavirus pandemic, no one knew exactly how the virus spread. People scrubbed their groceries. Governments urged people to stay home, wash their hands frequently and not touch their faces.

Masks quickly became a point of confusion when public health officials first discouraged people from wearing them by citing shortages and then advocating them. Mask mandates became a focal point in the Culture Wars as states, counties, and cities across the country passed a patchwork of guidelines.

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

For example, public health guidelines on masking in the US have shifted since the pandemic began.

February 2020

“Seriously Folks – STOP BUYING MASKS!” The then general surgeon, Dr. Jerome M. Adams, wrote on Twitter in February 2020: “You are NOT effectively preventing the general public from getting #coronavirus, but if healthcare providers cannot 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 not to catch or spread the coronavirus is to wash their hands frequently and stay home when they feel sick.

At the time, masks – especially N95s, which are thicker, fit tighter around your mouth and nose, and block smaller particles than surgical masks – were in great demand, which led to price cuts. There have been numerous bottlenecks in hospitals across the country.

Even Dr. Anthony S. Fauci, the director of the National Institute of Allergies and Infectious Diseases, expressed concern at the time that the urge by Americans to wear masks could lead to an even worse shortage of medical masks, including N95.

“They don’t want to take masks away from healthcare providers who are at real and present risk of infection,” said Dr. Fauci to CNN.

On March 15, the CDC made no mention of masks when recommending that gatherings in the United States – including weddings, festivals, parades, concerts, sporting events, and conferences – be restricted to 50 people.

April 2020

In April, officials reversed course, and the CDC urged all Americans to wear a mask outside of their homes to complement other public health measures like social distancing and hand washing.

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

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 forces with other airlines to urge passengers and flight attendants to wear face covers.

September 2020

Many officials have highlighted the public health benefits of masks. In September, Dr. Robert R. Redfield, then director of the CDC, told a Senate committee that masks are “the single most important and effective public health tool we have in combating the pandemic,” adding that the universal use of face coverings has consequences could have pandemic under control in months.

Updated

April 27, 2021, 5:19 p.m. ET

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

However, Mr Trump was quick to reject these comments, saying that Dr. Redfield “made a mistake” when he suggested that masks could be more useful than a vaccine.

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

January 2021

President Biden used his executive powers in January to impose mask requirements wherever he could, including on federal property and while traveling internationally.

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

March 2021

In March, almost exactly a year since the pandemic first terrified Americans, the CDC said people fully vaccinated against the coronavirus could gather in small groups indoors without masks or social distancing. Vaccinated adults could start planning mask-free dinners with vaccinated friends, the agency said.

March 2021

As vaccinations increased, some states began to lift mask mandates. Others, including Florida and South Dakota, never had one.

The Republican Greg Abbott from Texas lifted the mask mandate and capacity limits for all companies from March 10th. The order ensured that “all Texas businesses and families are free to determine their own destiny,” 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. Indiana Governor Eric Holcomb, a Republican, would follow suit in April, replacing a nationwide mask mandate with an advisory mandate.

Other states remained strict: in Massachusetts, for example, external masking was still required even when no one else was around.

April 2021

On April 27, the CDC announced that fully vaccinated people generally no longer need to wear masks outdoors and should continue to wear them during indoor gatherings or at crowded outdoor events. People who haven’t captured their shots can do without a mask in small outdoor gatherings as long as they’re with fully vaccinated friends and family members, the agency said.

Vaccinated adults should continue to wear masks and be at least three feet away from others in large public spaces – such as at outdoor performances or at sporting events or in shopping malls and cinemas – where other people’s vaccination and health status would be unknown, the agency said. And they should still avoid medium and large gatherings, crowds and poorly ventilated rooms, officials said.

A growing body of research shows that the risk of spreading the virus outdoors is far less than indoors. Viral particles spread quickly outdoors, according to public health authorities, so the risk of transmission is much lower, if not impossible.

“I think it’s pretty reasonable now that the risk in the open air is really, really small,” said Dr. Fauci on Sunday in “This Week” on ABC. In particular, “if you are a vaccinated person and you wear a mask outdoors – I mean the risk is obviously tiny.”