Confused by guidelines on quarantine after vaccination? You're not alone

The new guidance only applies if it has been at least two weeks since your final dose, no more than three months since that dose, and provided you don't develop symptoms.

"It's important to note that CDC is not suggesting someone who is vaccinated cannot spread COVID-19 within the first 90 days of being fully vaccinated, nor are we suggesting that expected protection from COVID-19 vaccines wears off after 90 days," CDC spokeswoman Kristen Nordlund told CNN by email. "The three months aligns with what CDC currently recommends for persons with natural infection, and we will continue to evaluate this period as we get more information on duration of protection from the vaccines."

Confused? Don't worry, you're not the only one.

"There is currently limited information on how much COVID-19 vaccines might reduce transmission or how long the duration of protection lasts," Nordlund said.

"However, we know that quarantine can be very disruptive to the individual as well as society. The benefits of avoiding unnecessary quarantine, specifically for those who are fully vaccinated, likely outweigh the unknown risks of transmission from a vaccinated person."

Plus that doesn't mean you can stop wearing a mask, keeping your distance from others, and following other CDC guidance. You're also not exempt from testing requirements for those returning from abroad.

While the guidelines imply that vaccinated people may be less likely to transmit the virus, the CDC makes clear that vaccine trials have largely focused on preventing symptomatic cases of Covid-19. That doesn't mean people can't catch the virus and spread it asymptomatically. But that's significantly harder to measure, experts say.

"There's not a perfect measure for describing how likely someone is to transmit the virus," said Dr. Megan Ranney, associate professor of emergency medicine at Brown University.

Measuring transmission indirectly

Directly measuring transmission of Covid-19 is difficult, so researchers use proxies to estimate how likely a person might be to transmit the virus.

One potential proxy is viral load -- how much virus people have circulating in their bodies. Research has shown that people with lower viral loads are less likely to transmit the virus.

A recent study in Israel found that people who had been infected 12 to 28 days after their first dose of the Pfizer vaccine had viral loads that were four-fold lower than if they were infected in the first 12 days. While the study had not been peer reviewed and doesn't include data after the second dose, Ranney said the findings were "really exciting."

"It suggests that you're less likely to pass it on to others even after the first weeks -- even before you've been fully immunized," she said.

Another study on the AstraZeneca vaccine also suggested that it could affect transmission, but it did so using a different measure. The researchers collected nasal swabs from trial participants in the UK every week and found that the rate of positive tests fell by half after two doses of the vaccine.

Neither of these studies measured transmission directly -- for example, by tracing contacts of study volunteers to see whether they became infected. But they give a positive sign of what experts have suspected for some time, based on experiences with vaccines for other diseases.

"It assumes, I think, that the vaccines are also interrupting asymptomatic transmission" and preventing the virus from replicating in people's noses and mouths, said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

"I think that's probably true," he said, adding, "I don't know that we have all the i's dotted and the t's crossed."

Limited data, limited recommendations

More studies that test vaccinated people regularly for Covid-19 will give doctors a better understanding of how vaccines impact transmissibility, and for how long, Ranney said.

That's partly why the recommendation only applies up to three months -- because the CDC and vaccine makers don't have much data going out any longer than that after people have been vaccinated. "Certainly vaccines will last more than three months," Hotez said.

CDC made similar updates to their quarantine guidelines back in August, when it said people who have recovered from Covid-19 in the past three months do not have to quarantine or get tested again as long as they do not develop new symptoms. According to the agency, "available evidence suggests that most recovered individuals would have a degree of immunity for at least 3 months," and reinfection appears to be uncommon during that time.

But the risk is not zero. Still, despite the uncertainties, Hotez and Ranney said the new guidance is a step in the right direction.

"In some ways, it's a welcome recommendation because at some point, we've got to start working towards normalcy and opening up the country," Hotez said.