Update: The Centers for Disease Control and Prevention has issued new advice for those who have been fully vaccinated. The most recent guidelines can be found here: Fully vaccinated people can visit with nearby grandchildren, dine indoors with one another, CDC says.

As the coronavirus vaccines have rolled out, so too have promising messages about what the shots mean for the countless lives upended by the pandemic.

Superspreader events are the leading cause of coronavirus transmission in the U.S. Here’s what they entail, and why they are so dangerous. (Adriana Usero/The Washington Post)

But while the vaccines are a critical step toward slowing the spread of a virus that has now caused more than 2 million deaths worldwide, killing hundreds of thousands in the United States alone, experts have repeatedly emphasized that getting vaccinated doesn’t mean an immediate return to pre-pandemic life.

“There are many people that think it’s kind of an antidote to it all and that once you’re vaccinated, you won’t have to mask or distance or any of those things,” said Namandje Bumpus, a professor of pharmacology and molecular sciences at Johns Hopkins University, who has participated in community calls about the vaccines. “Certainly, all of us getting vaccinated moves us toward that more quickly, but it’s not something that we’re going to be able to do as soon as we get vaccinated. We’re going to have to continue to be diligent the way that we have been.”

So far, more than 1.6 million people in the United States have been fully vaccinated, according to The Washington Post’s tracker. But public health officials say at least 70 percent of the population needs to be inoculated for the country to achieve herd immunity and stop the virus’s spread.

And with the virus continuing to spread rapidly across much of the country, many forms of in-person socializing carry some level of risk, including gatherings among people who are fully vaccinated, said Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital in Boston.

“I feel like a gathering of a small number of people where everyone is vaccinated is a much safer situation — much — than it was before we had vaccines,” Sax said. “The only thing that people want to hear, though, is, ‘Is it 100 percent safe?’ And we don’t have proof of that yet.”

Bumpus agrees. “Based on science and how vaccines work, it certainly is likely that that will end up being lower-risk. But right now, we just don’t know.”

Here is why experts say vaccinated people should still follow coronavirus safety measures at least for the time being — even around others who have been vaccinated.

Protection isn't immediate or guaranteed

Clinical trials have shown that the Pfizer-BioNTech and Moderna two-dose vaccine regimens are both highly effective at preventing illness from the virus, but they don’t provide instant and complete protection, said Onyema Ogbuagu, the principal investigator for Yale University’s Pfizer vaccine trial. “There’s no vaccine that I know that protects you the same day you get it,” Ogbuagu added.

It typically takes a week after the second dose for the Pfizer-BioNTech vaccine to reach 95 percent efficacy. Moderna’s vaccine efficacy increases to 94 percent two weeks after the second shot. This means that even if you get your two shots as scheduled and wait the appropriate amount of time, there is still a small chance you could be infected and develop symptoms, Ogbuagu said.

“On a population level, 95% efficacy still translates to 5/100, or 50/1,000, or 500/10,000 vaccinated persons still being vulnerable to symptomatic disease and maybe even more having asymptomatic carriage,” he wrote in an email.

The big unknown: Transmission

There are still unanswered questions about whether vaccinated people can transmit the virus — a major concern among public health and infectious-disease experts as the country grapples with a surge in infections and a sluggish vaccine rollout.

“Experts need to understand more about the protection that covid-19 vaccines provide before deciding to change recommendations on steps everyone should take to slow the spread of the virus that causes covid-19,” the Centers for Disease Control and Prevention notes in its vaccination FAQ. “Other factors, including how many people get vaccinated and how the virus is spreading in communities, will also affect this decision.”

Scientists are still trying to gather more data on how the vaccines affect transmission. It is possible that people who are vaccinated can be exposed to the coronavirus and become unknowing carriers, said Joshua Barocas, an infectious-disease physician at Boston Medical Center. People with no symptoms transmit more than half of all cases of the coronavirus, according to findings from a CDC model published this month.

The virus can also sometimes just hang around in a person’s nostrils after they are exposed, Barocas said. Then, all it takes is an ill-timed sneeze to potentially transmit it.

“What we’re trying to do is break the chains of transmission,” he said. “If you and I are both wearing a mask and social distancing when we’re sitting together, what it means is even if I’m carrying the virus with me, it dies with me.”

Calculating risk

Although gatherings of vaccinated people are likely to be lower-risk than those of unvaccinated people, such scenarios have many caveats, said Aditya Shah, an infectious-disease expert at the Mayo Clinic in Rochester, Minn. How well are the vaccinated people following public health measures in their daily lives? Does anyone live with a vulnerable individual who could have a greater chance of developing a severe infection?

“The problem with this virus is that it’s incredibly contagious and it’s very easily transmissible,” Shah said. “We’ve seen cases of very minimal exposures and then people getting infected from there.”

Although there are specific situations in which people may know that they have a very small chance of passing the virus to anyone else, calculating risk for any in-person interaction during the pandemic can often take “a lot of human brainpower and a lot of understanding your entire social network that many people don’t have really the time or capacity for,” Barocas said.

“It’s just hard to really know what the extent of your impact and your bubble actually is,” he added.

Temporary measures

Still, experts are encouraging people to remain hopeful and prioritize getting vaccinated, emphasizing that the recommendations to keep following public health measures are temporary.

“We all are tired and are at risk of burnout from all of this,” Barocas said. “The goal is not to say, ‘You absolutely cannot meet with people.’ It’s to say, ‘Let’s continue to do the things that we know lower your risk as best as you possibly can.’ ”

Barocas added that experts are “humbly asking for a little bit more time to be able to give a good recommendation that things can change.”

Research aimed at better understanding the publicly available coronavirus vaccines is ongoing, said Robert Atmar, a professor of medicine in the Section of Infectious Diseases at Baylor College of Medicine in Houston. In the meantime, Atmar, who recently completed his term as a member of the CDC’s Advisory Committee on Immunization Practices, urged people to think about protecting themselves and others.

“In the short term, life is going to look much the same until much more of society has had a chance to be vaccinated,” Atmar said. But, he added, those who got their shots have “taken a step to get us all closer to that light at the end of the tunnel and getting back to some sense of normalcy.”