What’s up with the 90 day limit?

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

The new quarantine exemption applies to those who:

• You have already taken both doses of the vaccine.
• They took the last dose between two weeks and three months ago.
• They have no symptoms of coronavirus infection.

“It is important to note that the CDC is not suggesting that someone who has been vaccinated cannot spread COVID-19 within the first 90 days after being fully vaccinated, nor are we suggesting that the expected protection of COVID-19 vaccines will disappear after 90 days,” CDC spokeswoman Kristen Nordlund told CNN. “The three months are in line with what the CDC currently recommends for people with natural infection, and we will continue to evaluate this period as we have more information on the duration of vaccine protection.”

She added: “There is currently limited information on how much COVID-19 vaccines can reduce transmission or how long protection lasts. However, we know that quarantine can be very harmful to the individual and also to society. The benefits of avoiding unnecessary quarantine, specifically for those who are fully vaccinated, are likely to 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, keeping their distance from other people and following other CDC guidelines. They are also not exempt from testing requirements when returning from abroad.

Can asymptomatic and fully vaccinated people transmit the virus?

Although the guidelines imply that vaccinated people are less likely to transmit the virus, the CDC makes it clear that vaccine testing has largely focused on preventing symptomatic cases of Covid-19. This does not mean that people cannot catch the virus and spread it asymptomatically, but it is significantly more difficult to measure, experts say.

Measuring Covid-19 transmission directly is difficult, so researchers use proxies to estimate how likely a person is to transmit the virus.

A potential proxy is viral load – the amount of viruses that 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 were infected 12 to 28 days after the first dose of the Pfizer vaccine had viral loads four times less than if they had been infected in the first 12 days. Although the study was not peer-reviewed and does not include data after the second dose, the results were considered “really exciting” by Megan Ranney, associate professor of emergency medicine at Brown University.

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

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

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

“This assumes, I think, that vaccines are also disrupting 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 it’s probably true,” he said, adding: “I don’t know if we have all the dotted i’s and the crossed t’s.”

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

This is partly why the recommendation only applies for up to three months – because the CDC and vaccine manufacturers don’t have much data coming out after people have been vaccinated. “Certainly vaccines last more than three months,” said Hotez.