It may seem counterintuitive, but health officials say that even after getting vaccinated against Covid-19, people still need to practice the usual pandemic precautions, at least for a while.
This means they still need to steer clear of crowds, continue to wear a good mask in public, maintain six feet or more of distance from people outside their household and frequently wash their hands, reports National Public Radio (NPR).
Here is a discussion with infectious disease specialists to get a better understanding of why.
Why continue with precautions after vaccination?
In the short run, it will take some time for the vaccine's effectiveness to build up.
With the Pfizer-BioNTech vaccine, a study published in The New England Journal of Medicine in December found that protection does not start until 12 days after the first shot and that it reaches 52% effectiveness a few weeks later.
A week after the second vaccination, the effectiveness rate hits 95%. In its application for authorization, Moderna reported a protection rate of 51% two weeks after the first immunization and 94% two weeks after the second dose.
"That's not 100%," says Paul Offit, director of the Vaccine Education Center and a member of the Food and Drug Administration's vaccine advisory board. "That means one out of every 20 people who get this vaccine could still get moderate to severe infection."
Can one spread the virus even if fully vaccinated?
Scientists studying the shots' effectiveness do not have an answer yet. And for public health experts, that lack of knowledge means one should act like the answer is yes.
Here is why -- before approving the Moderna and Pfizer vaccines, the FDA asked the vaccine manufacturers only whether their products protect people from Covid-19 symptoms.
They did not ask if the vaccines stop people who have been vaccinated from nevertheless spreading the virus to others.
Also read - Experts: Covid-19 vaccine roll-out won't achieve herd immunity this year
The emergency authorizations by the FDA that have allowed distribution of the two new vaccines cite only their ability to keep the person vaccinated from becoming severely sick with Covid-19.
In the words of the Centers for Disease Control and Prevention: "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."
How can the virus spread after vaccination?
All the Covid-19 vaccines and vaccine candidates under consideration for use in the US rely on bits of genetic material or virus protein — not anything that could grow into an active SARS-CoV-2 virus, the virus that causes the disease Covid-19.
The concern instead with the Covid-19 vaccine is about whether one might still have an asymptomatic infection despite immunization — without symptoms, but able to shed virus.
Here's how that might work – a vaccinated person encounters SARS-CoV-2. He or she is much less likely to develop symptoms — that is clear.
However, their immune system may not fight off the virus completely — it might allow some viruses to survive and reproduce and get expelled from the nose or mouth in a breath, cough or sneeze.
Why didn't the FDA demand information on infectivity before authorizing the first two vaccines?
Dr Larry Corey is a virologist at the Fred Hutchinson Cancer Research Center in Seattle and helps lead the Covid-19 Prevention Network, a federally funded consortium of vaccine experts who have been testing vaccines.
"We wanted to get an answer quickly," said Corey, and to do so required as simple an experimental protocol as possible. "You can't hang a lot of bells and whistles on a trial," he added.
Even though the pre-authorization studies of the Pfizer and Moderna vaccines were as streamlined as possible, they still required quite a lot of work. And in the midst of the pandemic, speed was of the essence.
So how to get an answer to whether vaccinated people can infect others?
Researchers are checking now on the folks vaccinated in the Pfizer and Moderna trials for antibodies showing that they have been infected.
It is not a perfect measure, Corey notes, because the antibody tests are not always accurate and antibodies can be transient.
Moderna officials have hinted that their vaccine prevents some shedding, but they have not been specific on how much. Some people who volunteered for the vaccine studies will be checked for evidence of the virus itself.
If someone is found to be shedding the virus, then researchers will check to see if that virus is capable of reproducing itself and, if it is, whether there's enough to infect other people. That raises another unknown: It is not yet clear what an infectious dose would be.
Nailing down the answers to all these questions is a laborious process. And no one is saying yet how soon the results of these sorts of studies of the Pfizer or Moderna vaccines will be available.
There are other ways to get at the answer. The Covid-19 Prevention Network has proposed an experimental study that would involve vaccinating more than 20,000 college students with Moderna's vaccine, checking them multiple times per week for virus shedding and doing contact tracing when infections occurred. The trial, which would cost several hundred million dollars, has so far been turned down by federal funders.
Maybe one of the vaccines that are in the pipeline would be a better idea?
While vaccines currently being tested are different in design from the Pfizer and Moderna products, scientists say there is no biological reason to think those vaccines will be more or less protective against shedding.
Once vaccinated, can masks be taken off in a crowded room where everyone else is vaccinated?
One might think they are home free in that case, but not yet. Remember that the Pfizer and Moderna vaccines are not 100% effective, and many in the research community still advise caution.
Once most people are protected and there's less virus circulating in the air — and less circulating in the community — the advice and restrictions on this may ease up a bit.
Will the new variant from UK make a difference in terms of spreading the virus after vaccination?
The newly identified variant, B.1.1.7, has not been around long enough to say for certain that the new vaccines are effective against it, but scientists are not too worried about that — lab studies suggest the vaccines will be protective against this strain.
However, infectious disease specialists are concerned that any strain that is more contagious — which B.1.1.7 clearly is — might quickly increase the number of Covid-19 cases in the world.
So what's the bottom line?
With cases and deaths surging throughout the US, the people who are treating Covid-19 patients really want everyone to continue to wear a mask, keep distance and wash hands, even if they have been vaccinated, until the research on shedding has yielded some answers.
Dr Carlos del Rio of Emory University said: "It's not like you'll need to wear a mask for the rest of your life. You need to wear your mask until we have the data, and we're trying to get the answers as fast as we can.”
