What we know and don’t know about how Covid-19 vaccines slow the spread of the virus.
After months of exhausting isolation, widespread economic pain, and an extraordinary toll on human life and health, several Covid-19 vaccines are here.
Surely, this means we can stop wearing a face mask?
As with so much else in this pandemic, there isn’t an easy answer for exactly when we can start to relax. But, clearly, the rapid rollout of Covid-19 vaccines, like the ones developed by Moderna and Pfizer/BioNTech, is our best path out of the crisis.
“We have every reason to believe that these are among some of the very best vaccines that we have ever tested,” said Dr. Aaron Richterman, a fellow researching infectious diseases at the University of Pennsylvania.
Though clinical trials give us confidence that the Pfizer/BioNTech and Moderna vaccines block close to 95 percent of cases of the disease — thereby preventing the most severe outcomes of Covid-19 — there are still some uncertainties. Key among them is how well vaccines work to block transmission of SARS-CoV-2, the virus that causes Covid-19. Vaccines serve not only to protect individuals but also, after a certain threshold of vaccination, the population as a whole. That threshold is herd immunity — where even people who haven’t been vaccinated or infected before are protected because so many of the people around them are immune.
Transmission also has important practical consequences for the risks that arise as vaccinated individuals interact with everyone else, whether that’s in public parks, schools, households, or health care facilities. Early evidence points toward vaccines reducing transmission of Covid-19, but by exactly how much remains unclear. And that’s stirred up a fierce debate lately around how cautious we should be when talking about the power of the vaccines.
The question, given what scientists know and don’t know, is what message should people get about Covid-19 vaccines and how should they behave when they get them?
It’s a difficult needle to thread, to convey both optimism and caution, and there’s disagreement among scientists and experts over what should be the selling point of vaccines in the current moment. If you’re thinking about how your life might change after you get your shots, here’s what to consider.
What we know about Covid-19 vaccines and transmission
The main problem is that while the Covid-19 vaccines that are now available are amazingly effective at protecting recipients, it’s not clear how much they can prevent them from spreading the virus to other people. And because of that uncertainty, along with the current levels of spread of the disease, public health guidance still calls for the immunized to maintain social distance and wear face masks.
In the meantime, research is underway to figure out by how much vaccinated people can transmit the virus. During phase 3 clinical trials, the main thing researchers are looking for is how well vaccines prevent disease — that is, people getting infected and showing detectable symptoms like fever, coughing, shortness of breath, and a loss of taste or smell.
However, asymptomatic transmission has emerged as a major driver of Covid-19. Getting a handle on how much asymptomatic transmission can occur even with a vaccine requires mass testing to detect the virus since there are no other outward signs of infection. For clinical trials with thousands of participants, testing is a tedious, time-consuming endeavor, and there aren’t many robust findings yet.
That said, there is emerging evidence that Covid-19 vaccines do slow transmission.
Moderna, for example, screened its trial participants for SARS-CoV-2 between the first and second doses of its vaccine, finding that two-thirds fewer people in the vaccine group tested positive for the virus compared to the placebo, according to the company’s briefing to the Food and Drug Administration in December. It suggests that some asymptomatic infections start to be prevented after the first dose.
During the clinical trials for the Covid-19 vaccine developed by AstraZeneca and the University of Oxford, which has not been approved in the US, researchers tested participants more frequently. An early analysis showed that the vaccine may be 59 percent effective at stopping asymptomatic infection.
There are other signs indicating the vaccines can reduce spread. Changes in immune system markers like antibodies in people who are vaccinated comport with what scientists expect in a situation that prevents the virus from setting up shop in people’s airways.
Monica Gandhi, an infectious disease doctor and a professor of medicine at the University of California San Francisco, said the focus on transmission in the context of Covid-19 vaccines can be misleading when comparing them to other vaccines. Part of what’s skewing the picture is that we have more information about Covid-19 transmission dynamics than other respiratory infections. “We never do mass testing for any respiratory virus unless you don’t feel well,” Gandhi said.
There are lessons we can draw from other vaccines, too. Researchers say it’s highly unlikely that a vaccine with a high efficacy against disease wouldn’t also make a significant dent in transmission. In fact, there are vaccines that are given mainly to prevent transmission more so than the disease, like the rubella vaccine, according to Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital and a professor at Harvard Medical School.
With Covid-19 vaccines, the fact that they prevent the most severe outcomes — even if they don’t prevent every instance of disease — can reduce transmission by itself. People with milder symptoms are less likely to cough and spread the virus through the air to others. “Even if it shifts it from symptomatic Covid to asymptomatic Covid, that still is a win for transmission because asymptomatic people are less likely to transmit it because they don’t have as much virus for as long,” Sax said.
On the other hand, there are other vaccines that can prevent disease but have a much weaker effect on transmission, like the pertussis, or whooping cough, vaccine.
For the most part, the evidence is pointing toward Covid-19 vaccines reducing transmission of the virus. The critical question is by just how much, since that will shape the point where herd immunity occurs. “The exact amount that it reduces asymptomatic transmission is going to have consequences,” Richterman said. The answer will likely emerge in the coming months as researchers gather more data from clinical trials as well as among vaccine recipients in the general population.
What message do we send in the moment?
So scientists generally agree: The vaccines are essential for ending the pandemic, though they will take weeks or months to blunt the spread of Covid-19 across the population. Until that time, it’s necessary to keep up mask-wearing and social distancing in public.
But it’s not as though one day the country will cross a line going from unsafe to safe; rather, there will be a decline in risk over time. “I think the better way to frame it is the vaccine is going to make every single activity the person does safer,” Richterman said. And while scientists can measure risk and come up with tactics to reduce it, they can’t determine how much risk is tolerable. That’s a value judgment people have to make as individuals.
Vaccines are certainly a major risk-reducer, arguably the largest when it comes to Covid-19. The risk of infection and transmission doesn’t drop to zero with a pair of shots, but when combined with other measures like wearing face masks, they become a firewall against transmission. Right now, though, in the context of uncontrolled spread of Covid-19, even a reduced risk of transmission could still lead to problematic levels of new infection.
And there’s still a long way to go. Even though upward of one-third of the US population may have already been exposed to the virus, we don’t fully know who has had it because there are so many asymptomatic cases and because of gaps in testing. It’s also not clear how long immunity lasts after infection and how well it will hold up against new SARS-CoV-2 variants, although early evidence shows immunity does last at least a few months and that prior infections offer some degree of shielding against newer versions of the virus. So the transmission aspect of the pandemic is going to remain a major issue for some time.
“My biggest concern right now in the short term is getting people to make sure they’re not easing up on the precautions they need to take, given the current situation and the lack of vaccine availability,” said Angela Rasmussen, a virologist at Georgetown University.
One of the most delicate times in the pandemic will be the period between its worst throes and widespread immunity. That’s when there will be large groups of people who are vaccinated as well as those who are not vaccinated interacting in the same public spaces but with very different risk exposures.
Though some groups have broached the idea of using vaccine passports to identify the immune, there is no easy way to tell whether someone is protected just by looking at them, so across-the-board pandemic restrictions will likely have to remain in place. That may prove frustrating for people who survived the pandemic and went through the trouble of getting immunized but still can’t relax.
The message to those vaccinated people in this twilight period of the pandemic must be that they are duty-bound to keep up precautions like wearing masks in order to protect others as an act of social solidarity.
But what’s the best way to frame this? Are we in the home stretch of the pandemic, or are we still mired in the worst phase? Should health officials emphasize how vaccines will return everyone to normal or highlight the unknowns and counsel caution? Should the vaccinated be scolded if they start to hang out with friends and travel?
Rasmussen noted that with the uncertainties around the Covid-19 vaccines, as impressive as they are, there is a fear of overpromising and underdelivering. The final results could reveal that vaccines may not block transmission as much as hoped, so if they’re overhyped, trust in public health officials could erode and lead to more vaccine hesitancy.
On the other hand, as groups of people get vaccinated, they might be able to relax around each other as their collective risk declines. Members of a household, neighbors, or people living in long-term care facilities may be able to share the company of others who are also immune.
But even vaccines, masks, and social distancing together won’t stop spread due to reckless behavior, just as airbags, seatbelts, and crumple zones don’t mean that it’s safe to drive inebriated over the speed limit. Vaccines are not a license to resume crowded indoor gatherings since the overall vaccination rate is still low and the spread of the virus is still high.
“You can potentially get together with your parents that you haven’t seen in a long time if all of you have been vaccinated,” Rasmussen said. “What you should not do is get together with a bunch of your vaccinated friends and go hit the bars.”
Gandhi agreed that precautions will be necessary in many circumstances for people who receive vaccines.
“My risk tolerance is I will wear a mask around those who are unvaccinated,” Gandhi said. “I think many doctors will take off their mask around vaccinated people. And only vaccinated people.”
However, she argued that the messaging emphasis should be on how vaccines will speed up the return to a world outside the pandemic. Belaboring the blank spaces in our understanding of them when there is so much good news could also create hesitancy and undermine progress. Without a sense of progress and an achievable goal, it may get harder to keep up precautions until there is widespread immunity.
“It’s very helpful to tell the public that someday the masks will come off,” Gandhi said. “You can say it will be longer [to get to the end of the pandemic], and it will be, but please keep giving people hope.”
How can we tell we’ve crossed the finish line?
The main benchmark for ending the pandemic and the goalposts of a vaccination campaign should be to reduce fatality rates. “We should go all-in for mortality. The first thing we should see is a substantial, substantial reduction in mortality in the population,” said Saad Omer, director of the Yale Institute for Global Health. “Even if we don’t find out that there is reduction in transmission, if enough people are protected and mortality goes down drastically … even if it’s just individual effects, that’s a good way of returning to normal.”
Such a scenario would downgrade Covid-19 from a lethal public health threat to a moderate concern, and perhaps even a nuisance. Until there is a dramatic decline in fatality rates, however, face masks and social distancing will still be a part of everyone’s life, including those who are vaccinated.
After that, measures of transmission, such as the fraction of Covid-19 tests that yield positive results, could be used as an indicator of how much spread is still occurring.
The US may have to contend with sporadic outbreaks and even vaccine boosters, as immunity declines and new variants of the virus emerge. But widespread immunization creates a scenario where many of the most onerous burdens of the pandemic can be lifted.
Given the pace of progress in vaccinations, that could happen later this year.
“If there are no crazy variants, we can be in a situation where in fall things are more normal,” Omer said. “Maybe not fully normal, but better.”
Author: Umair Irfan