For example: “Indoor visits or small gatherings likely represent minimal risk to fully vaccinated people.”

On Wednesday, CDC director Rochelle Walensky said she could not give a definitive answer to what a “small” gathering is, because there are too many variables.

“If we define a small- and medium-sized gathering, we actually also have to define the size of the space that it’s in, the ventilation that is occurring, the space between people. And so, I think we should get back to the the general concepts,” Walensky said.

The situation has left people where they’ve been since the start of the pandemic: forced to play the role of amateur epidemiologist.

In the early days of the pandemic, we wondered if we could catch the coronavirus from a passing jogger and if our groceries, fresh from the store and resting on the kitchen counter, threatened to kill us. Science has attenuated some of our earliest fears. But more than a year into this crisis, we’re still trying to perform complicated risk calculations while relying on contradictory research and shifting CDC guidance.

Risk analysis is not something humans are necessarily good at. We rely on anecdotes more than scientific data. The questions we ask rarely have a simple yes or no answer. Risk tends to be on a sliding scale. Outside of self-isolation, there is no obvious way to drive the risk of viral transmission to zero, nor is risky behavior guaranteed to result in a dire outcome. We have no choice but to live probabilistically.

The risk landscape keeps changing as well. The virus is mutating, and there are many different variants in circulation. Many people are now fully vaccinated, some only partially vaccinated (in between shots, for example), some unvaccinated and some armored with a level of immunity through natural infection. Add the extreme variation in disease severity because of age and underlying conditions, and the risk equations get so long we may run out of chalkboard.

The restrictions imposed by governments have sometimes made little sense. Casinos were open before schools in some states. Mask mandates outdoors remained in place even when indoor dining became permitted.

“It seems to me if we are going to have indoor dining, we should have mask-free jogging,” Harvard epidemiologist Marc Lipsitch said in an email.

One thing that is incontrovertibly true: The coronavirus vaccines are remarkably safe and effective, and people should get vaccinated if possible.

“These are off-the-scale good,” said Amesh Adalja, an infectious-disease doctor and senior scholar at the Johns Hopkins Center for Health Security. “These are much better than vaccines that we rely on every year, like the flu vaccine.”

Even for people sold on vaccines, there remain lingering questions about what is and isn’t safe, and what is and isn’t the proper way to go about daily life in an increasingly vaccinated society. Here, we present some answers, with the caveat that our knowledge of the coronavirus, SARS-CoV-2, is still evolving, as is the virus itself.

Q: Why do I still need to wear a mask after I’m fully vaccinated?

A: You don’t need to wear a mask outdoors when fully vaccinated, except in crowds (such as at a sports stadium or a concert), nor do you have to wear one indoors among other vaccinated people or members of your own household.

But there are situations where you still need to mask up. You could still get infected with the coronavirus, and although it would most likely be mild or asymptomatic, you could transmit the virus to another person. Again, the odds of that happening are low, and there is encouraging data from Israel that suggests vaccinations dramatically reduce community spread.

But remember: A vaccination campaign is not simply about protecting the vaccinated individual. The goal is to build immunity broadly. Moreover, many communities still require masks in public settings — so it’s the law. It’s also polite — you don’t want to make people guess if you’ve been vaccinated or not. That probably will change when infection rates plummet and vaccinations are far more widespread.

“It is also a show of solidarity that we are still in this together,” said Maria Van Kerkhove, technical lead for the World Health Organization’s covid-19 response. “It’s about you and your community, your family, your friends, your workplace, your loved ones. It’s not just about you.”

At some point, viral transmission will plummet. We’re a long way from that point. As long as the virus is circulating in our communities, we need to use what we can to limit the spread and drive down the infection rate.

“Because [the vaccines] are not perfect, that’s precisely why we are urging people to be cautious,” Surgeon General Vivek H. Murthy said in a recent White House covid-19 task force news briefing. “We have great confidence in vaccines. We understood they are not perfect.”

Q: If you’re vaccinated, are you definitely protected against the coronavirus?

A: You’re very likely protected from symptomatic illness. That’s why Adalja, echoing the consensus, said, “These vaccines are something that will change your life.”

In clinical trials, the Pfizer and Moderna vaccines were about 95 percent effective in blocking symptomatic illness after two shots. The one-shot Johnson & Johnson vaccine was not quite as effective but just as good at preventing severe illness and death — which is the highest public health priority in a pandemic like this.

Q: But aren’t there also breakthrough infections?

A: As of April 26, the CDC had documented 9,245 breakthrough infections among fully vaccinated people. But look at the denominator: Those cases were among more than 95 million people. That’s fewer than 1 in 10,000 people vaccinated. (The agency noted that this is probably an undercount because of lack of testing and surveillance.) Of those rare breakthrough cases known to the CDC, 27 percent were asymptomatic and only 9 percent required hospitalization.

Adalja said people need to focus on probabilities and not anecdotes.

“This is kind of a cognitive bias that people have with many kinds of risk. It’s just like when there’s a shark attack in Australia. How much coverage does that get?” he said.

Q: Should people who got the Johnson & Johnson vaccine worry about blood clots?

A: If you notice unusual and serious side effects, such as severe headaches, contact your doctor. But the risk is extremely low. Federal regulators reauthorized the use of the vaccine after a 10-day pause, having found 15 cases of a serious clotting disorder among the 7 million people who had received the vaccine at that time. By any calculation, the risk of a bad vaccine reaction is much less than the risk of getting a serious case of covid-19.

Paul A. Offit, a pediatrician at Children’s Hospital of Philadelphia who is an expert on vaccination, suggests that the Johnson & Johnson coronavirus vaccine suffers from bad timing. Had it been approved first, before the Pfizer and Moderna vaccines, its many virtues would have been celebrated and the rare side effects minimized.

He noted that the Johnson & Johnson vaccine is “refrigerator stable” for up to five weeks. The vaccine is appealing to public health officials because it’s one-and-done and can be more easily deployed in remote locations and in places where recipients are homebound.

Q: How long will natural or vaccine-induced immunity last?

A: No one knows, but the initial evidence is encouraging, said Alessandro Sette, a professor of immunology at the La Jolla Institute for Immunology. A research paper published by Sette and fellow researchers in January showed that 90 percent of people who recovered from a coronavirus infection had robust levels of immunity eight months after they became sick. Immunity did not suddenly drop after eight months — that was merely the limit of the research period.

“Ninety percent having a good immune response also means 10 percent don’t. That is a reason for vaccinating and being careful even if you had the disease,” Sette said.

Immunity post-vaccination also appears durable, and there is less variability in levels of antibodies and other immune system cells following a vaccination than following a natural infection, Sette said.

Because this is a novel disease, and vaccines have not been widely deployed for very long, it is too soon to know how long antibodies will last. But Sette pointed out that the immune system has other weapons against invasive viruses, including “killer T-cells,” which continue to be able to recognize infected cells and kill them, preventing viral replication.

Q: Do the vaccines work against these new virus variants? And shouldn’t we be worried about a new variant that has even scarier, vaccine-evading mutations?

A: The immune response generated by vaccines is sufficiently protective against coronavirus variants to prevent most people from getting seriously ill.

Infectious-disease experts do worry about future mutations that could allow the virus to exhibit vaccine evasion. That said, there are limits to how much the virus can mutate — how much it can change its structure — and still function, according to Sette.

“The virus has to walk a tightrope,” he said. The virus can mutate to escape the effect of a specific antibody, but “it can’t change too much.”

He added, “While the virus has surprised us this year in a number of ways, the data we’ve seen so far does not suggest there’s an infinite number of ways the virus can mutate and escape immune recognition and still be as infectious.”

Q: When will we reach herd immunity?

A: No one knows what level of immunity would throttle virus transmission, and it probably varies from one environment to another and from one season of the year to another. But in the United States, at least, vaccinations have already had an effect. The virus increasingly is slamming into immune-system walls. Eventually, with enough vaccinations, most of the people who get infected will be dead-end alleys for the virus.

The virus appears destined to pop up in smaller outbreaks that could be more easily contained. But the virus won’t disappear, especially because it continues to spread at catastrophic rates in many countries that have low levels of vaccination. The only infectious disease-causing virus ever eradicated is smallpox.

For now, successful navigation of the pandemic may simply mean taking steps to reduce the threat of a serious case of covid-19 (as best as anyone can determine it) to the level of other threats that we typically tolerate, and which don’t tend to keep us awake at night.

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Joel Achenbach covers science and politics for the National desk. He has been a staff writer for The Post since 1990.