The playbook comes after months of investigations on how the coronavirus spreads and affects the body. Scientists combined that with knowledge gained from years of experience managing occupational-health hazards in high-risk workplaces, such as factories and chemical plants, where tiny airborne pollutants can build up and cause harm. They say different types of workplaces—taking into account the types of interactions workers have—need slightly different protocols.

The safety measures have taken on new urgency in recent weeks as new infections, hospitalizations and deaths rise across the U.S. and Europe, and potentially more-transmissible variants of the virus spread around the globe. This phase of the pandemic is prompting a new wave of stay-at-home orders, closures and travel restrictions, important first steps to curbing contagion. Infection-prevention specialists say known strategies for stemming spread should continue to work against the new variants, but that increased adherence is even more important.

Vaccines are rolling out, but slowly, and access will be limited mostly to high-priority groups for some time.

“We have to still deal with ‘the right now.’ We’ve zeroed in on this set of controls that we know work,” said

Joseph Allen,

director of the Healthy Buildings Program at the Harvard T.H. Chan School of Public Health.

Over the past year, the lack of consistent and cohesive messaging among scientists and lawmakers has seeded confusion over what makes up risky behavior, what activities should be avoided and why. That is beginning to change as consensus builds and scientists better understand the virus.

In the U.S., scientists at first advised people against wearing masks, in part because of shortages, while the idea of stay-at-home orders received severe pushback from some lawmakers. Early in the pandemic, testing was limited to people with symptoms, also partly due to shortages. That advice has shifted, but a year later, sufficient testing remains a critical issue.

Countries such as New Zealand and others in Asia adhered to a combination of basic mitigation strategies from the start—particularly masking, large-scale testing and lockdowns that broke transmission chains. They have tended to fare better than those that didn’t.

In one of his first moves, President

Biden

signed executive orders to require masks be worn on federal property and at airports and other transportation hubs. The administration said it is focusing on increasing the availability of vaccines, and also stressed the importance of widely available testing, which still lags in low-income and minority communities.

The current scientific playbook follows from two of the biggest research insights since the start of the pandemic. First, individuals who aren’t showing symptoms can transmit the virus. Infectious-disease experts worry most about this silent spread and say it is the reason the pandemic has been so hard to contain. While visibly sick people can pass on the virus, data cited by the Centers for Disease Control and Prevention estimate that 40% to 45% of those infected never develop symptoms at all. With the new viral variants that can transmit more readily, the potential for silent spread is even higher, infectious-disease experts said.

Secondly, researchers now know that tiny airborne particles known as aerosols play a role in the spread of Covid-19. These can linger in the air and travel beyond 6 feet.

An early hallmark of the pandemic response focused on the risk of transmission through large respiratory droplets that typically travel a few feet and then fall to the ground. Businesses rushed to buy plexiglass barriers, creating shortages.

The barriers can be good at preventing larger virus-containing droplets from landing on and infecting healthy individuals. They may offer some protection in shielding workers who have brief face-to-face interactions with many people throughout the workday, such as cashiers and receptionists, some occupational-health experts said.

Yet in settings like offices, restaurants or gyms, the role of the barriers is murkier, because activities like talking loudly and breathing deeply create aerosols that can waft on air currents and get around shields.

Also, installing such barriers could affect airflow throughout the space, environmental-health experts said. It is possible they could limit proper ventilation, making things worse, they said.

“There seems to be an assumption that particles are going to get stopped by the barriers, which is simply not true,” said

Lisa Brosseau,

an industrial hygienist and research consultant for the University of Minnesota’s Center for Infectious Disease Research and Policy. Airborne particles ferrying the virus “really distribute all over the place.”

The emphasis on intense surface cleaning has diminished as scientists have come to understand that indirect transmission through contaminated surfaces doesn’t play as critical a role in the spread of Covid-19 as they thought in the early days of the pandemic. In September, the CDC published sanitation guidelines for offices, workplaces, homes and schools that said that, for most surfaces, normal, routine cleaning should suffice, and that frequently touched objects, such as light switches and doorknobs, should be cleaned and disinfected.

“Sanitation is important in general always,” said

Deborah Roy,

president of the American Society of Safety Professionals. “The idea is we went overboard at the beginning because of the amount of unknowns. Now, we’re in a situation where we have more information.”

Temperature checks have become less popular among some employers because scientists now know that not all Covid-19 patients get fevers. One large study published online in November in the New England Journal of Medicine showed only 13% of Covid-19 patients reported a fever during the course of their illness.

Scientists now understand that brief encounters with an infected person can lead to spread, according to an October case study—an advance from earlier, when the rule of thumb was to avoid close contact for 15 consecutive minutes or longer. The report urged people to consider not just time and proximity in defining close contact with a Covid case, but also ventilation, crowding and a person’s likelihood of generating aerosols. Following the report, the CDC changed its definition of close contact to a total of 15 minutes or more over a 24-hour period.

Fresh air and effective filters indoors are important because they can remove virus particles before they have time to infect.

Masks offer a similar benefit, by lowering the amount of particles that infected individuals emit. Some scientists say there could be a benefit to doubling up on masks, as a second layer may improve both filtration and fit, so long as the masks are worn correctly.

A study published in October found that in countries where mask wearing was the norm or where governments put in place mask mandates, coronavirus mortality rates grew much more slowly than in countries without such measures. This fall, the CDC said that masks also offer some personal protection by reducing a wearer’s exposure to infected particles.

The combination of airborne particles and personal interactions, even among people who don’t feel ill, can turn wedding receptions, plane rides and choir practices into superspreading, potentially deadly events.

“For Covid, those two factors—asymptomatic spread and aerosolization—is what made mask-wearing so essential,” said

Megan Ranney,

emergency physician and assistant dean at Brown University.

Lessons can be gleaned from an outbreak at a Canadian spin studio last fall. The operators of the SPINCO studio in Hamilton, Ontario, had many public-health measures in place, including limiting the number of bikes in each class and screening staff and attendees with a questionnaire about topics including symptoms and travel. Rooms were sanitized within 30 minutes of a completed class, and towels were laundered, according to a statement provided last fall by

Elizabeth Richardson,

medical officer of health for the city of Hamilton.

Masks were also required before and after workout classes, Dr. Richardson said.

In total, 54 people who attended workouts over a span of several classes became infected. Another 31 cases were tied to the outbreak after spin-class attendees who contracted the virus then passed it on. The spin studio temporarily shut down following the outbreak and later reopened. It is currently not offering classes due to local regulations that mandated the closure of all gyms and fitness centers amid rising Covid-19 cases in the area.

In a November statement following the outbreak,

Michelle August,

founder of SPINCO, said that the company has “always put safety first and [has] exceeded all recommended guidelines from public health throughout” the pandemic. She said SPINCO has also strengthened and heightened its Covid-19 mitigation measures. SPINCO’s website currently says face masks are mandated throughout workouts in the company’s Hamilton location.

It also says that SPINCO is installing air purifiers in all of its studios that filter air in the rooms every 17 to 21 minutes. Airborne transmission experts recommend that building managers pump in clean, fresh air between three to six times an hour and that they install filters that are proven to effectively trap and remove a substantial number of virus-carrying particles.

To film a stage play of “A Christmas Carol” in November, the Guthrie Theater in Minneapolis upgraded its air filters and increased the rate at which the ventilation system pumps in outside air, said

Brooke Hajinian,

the Guthrie’s general manager. Management staggered arrival times, and a compliance officer made sure everyone socially distanced, wore their masks properly and washed their hands.

The theater divided staff into pods depending on how close they must get to the lone actor on stage, who portrayed Charles Dickens and didn’t wear a mask while performing, according to Ms. Hajinian. Those working nearest the stage underwent testing three times a week and wore N95 masks at all times, she said, while cleaning and security crews, who didn’t interact with the stage crews, wore cloth masks and didn’t undergo testing.

Ms. Hajinian said she monitored the staff’s testing results and symptoms. “Any symptom is not a failure of this plan,” she said. Catching a case “and isolating it—that’s what success looks like for us,” she said. There were no cases, she said.

Scientists say multilayered safety efforts are needed because no single prevention method is 100% effective.

One of the largest studies of asymptomatic transmission to date showed that frequent testing was essential in identifying infections among a group of nearly 2,000 Marine recruits required to socially distance and wear masks except while eating and sleeping.

The study looked at cases identified with lab-based tests that search out and amplify the genetic material of the virus, but those tests aren’t as easily scaled as so-called rapid antigen tests, which search for viral proteins.

Results from lab-based tests can sometimes take days, while results from rapid tests are usually available in less than an hour. As a result, some epidemiologists have been advocating for widespread use of antigen tests to prevent outbreaks, because they are cheaper and don’t require high-tech laboratory equipment to run, meaning they can be deployed in a broader range of settings.

The shift toward using frequent, inexpensive and rapid tests on the same people multiple times a week to screen entire populations—instead of one-time tests on individuals who have symptoms—will be important to efficiently break transmission chains, epidemiologists said.

“Unless we’re doing really broad, frequent screening of the people at large, we’re completely missing the vast majority” of infections, said

Michael Mina,

an assistant professor of epidemiology at the Harvard T.H. Chan School of Public Health. “We have to change how we’re doing this.”

While rapid tests tend to be less sensitive than lab-based tests, Dr. Mina said the data suggest they have high sensitivity when people are most likely to be infectious.

Other infectious-disease experts have touted contact tracing to identify and bust clusters of infection. But they say the strategy works best when cases aren’t surging, as they are now. When transmission rates are too high, limiting gatherings, travel and crowding are more effective at denting spread, said

Abraar Karan,

a global-health physician at Brigham and Women’s Hospital and Harvard Medical School.

In places without big surges, a high-tech approach is becoming increasingly useful: genetic epidemiology, or tracking tiny changes in viral genomes to map out transmission chains. As the coronavirus replicates and moves from person to person, its genes change slightly. Sometimes, those tiny changes are unusual, and they can be particularly useful in mapping transmission events, according to

Justin O’Grady,

an infectious disease expert at the Quadram Institute in the U.K.

By sifting through the differences among more than 1,000 viral genomes, Dr. O’Grady and his collaborators found that a particular viral variant was moving through multiple nursing homes in the U.K., among patients and staff, but not among the wider community. The unpublished data suggested that transmission was facilitated by the movement of staff from one facility to another, Dr. O’Grady said. The team relayed the findings to government authorities and advised them to restrict staff moving among facilities during the pandemic.

“Sometimes genomic epidemiology is able to find hidden transmission links that traditional epidemiology would struggle to find,” Dr. O’Grady said. “We can’t stop transmission, but when we find a superspreader event…we can bring in the right prevention methods to stop it from spreading further.”

—Betsy McKay contributed to this article.

Write to Daniela Hernandez at daniela.hernandez@wsj.com, Sarah Toy at sarah.toy@wsj.com and Caitlin McCabe at caitlin.mccabe@wsj.com