What’s new with the 90 day limit?

(CNN) – Fully vaccinated people can now skip quarantine if they have been exposed to COVID-19, according to new guidance from the Centers for Disease Control and Prevention. But the agency acknowledges that many researchers ignore the impact of vaccines on transmission.

The new quarantine exemption applies to those who:

• Have received both doses of the vaccine.
• They received their last dose between two weeks and three months ago.
• Do not show symptoms of coronavirus infection.

“It is important to note that the CDC does not suggest that a vaccinated person cannot spread COVID-19 within the first 90 days after being fully vaccinated, nor that the expected protection against COVID-19 vaccines wears off after 90. days, ”CDC spokeswoman Kristen Nordlund told CNN. “The three months is what the CDC currently recommends for people with a natural infection, and we will continue to assess this period as we get more information on how long vaccine protection lasts.”

She added: “There is currently little information about how much COVID-19 vaccine might reduce transmission or how long the protection lasts. However, we know that quarantine can be very disruptive for the individual as well as for society. The benefits of avoiding unnecessary quarantine, especially for those who are fully vaccinated, likely outweigh the unknown risks of transmission from a vaccinated person. “

The quarantine exemption does not mean that fully vaccinated people can stop wearing a mask, stay away from others, and follow other CDC guidelines. They are also not exempt from the testing requirements when returning from abroad.

Can asymptomatic and fully vaccinated people transmit the virus?

While the guidelines imply that those vaccinated are less likely to transmit the virus, the CDC makes it 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 it’s much harder to measure, experts say.

It is difficult to directly measure the transmission of Covid-19, which is why researchers are using proxies to estimate the likelihood that a person can transmit the virus.

One potential proxy is viral load – the amount of virus circulating in their body. 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 were infected 12 to 28 days after their first dose of Pfizer vaccine had a viral load four times lower than if they were infected within the first 12 days. Although the study was not peer reviewed and does not include data after the second dose, the results were rated “really exciting” by Megan Ranney, associate professor of emergency medicine at the University. Brown.

“This suggests that you are less likely to pass it on to other people after the first few weeks – even before you are fully vaccinated,” she said.

Another study, on the AstraZeneca vaccine, also suggested that it might affect transmission, but it did so using a different measure. Researchers collected nasal swabs each week from trial participants in the UK and found that the rate of positive tests halved after two doses of the vaccine.

None of these studies measured transmission directly – for example, by tracing the contacts of study volunteers to see if they were infected. But they give a positive sign of what experts have suspected for some time, based on experiences with vaccines against other diseases.

“This assumes, I think, that vaccines also interrupt asymptomatic transmission” and prevent 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 if we all have the dotted i’s and t’s crossed.”

Other studies that regularly test people vaccinated for Covid-19 will allow doctors to better understand the impact of vaccines on transmissibility and for how long, Ranney said.

That’s part of why the recommendation only applies for three months – because the CDC and vaccine makers don’t have much data for longer than after people are vaccinated. “The vaccines will definitely last more than three months,” Hotez said.