Vaccine keeps you healthy, but will it stop the spread?
Data on transmission is scanty and answer is key to containing virus
The first vaccine for COVID-19 arrived last week in California, where it will protect an ever- growing slice of the population from the effects of the deadly virus. Its first doses should help alleviate staffing concerns at hospitals, as fewer workers call in sick, and save the lives of some of our most vulnerable, as residents of long-term care facilities gain immunity.
How much the vaccine will stem the spread of the coronavirus, however, remains an open question.
The two vaccines approved for use in the U.S. — Pfizer-BioNTech’s a week ago and Moderna’s on Friday — have both been shown to reduce the risk of serious disease from the vi
rus by about 95%. But Moderna provided only preliminary findings on how well its vaccine prevents coronavirus carriers from transmitting the virus, and Pfizer submitted none at all. Those who are vaccinated still may get the virus, and if they still can pass it on, that could prolong the pandemic.
“At the moment, we don’t know whether these vaccines will reduce transmission,” said Dr. Arthur Reingold, the chairman of California’s vaccine safety workgroup and division head of epidemiology and biostatistics at UC Berkeley. “But that’s quite plausible. It’s just that we really don’t have the data yet.”
Because of these unknowns, the Centers for Disease Control and Prevention advises even those who have already been vaccinated
to keep up what’s known to curb transmission. Washing hands frequently. Mask wearing. Shifting activities outdoors. Staying distanced.
Whether a vaccine prevents transmission as well as illness is a critical question each time one is developed, for any disease. What information there is about coronavirus vaccines has been promising, but there is precious little of it.
In the Moderna trials, participants who received the vaccine tested positive without symptoms about two-thirds less often that those who took the placebo, suggesting many fewer people had the ability to transmit the coronavirus. But participants were tested only before receiving their first dose and again before their second; no sample was taken after their second dose, leaving researchers with substandard data that didn’t meet the trial’s protocol.
AstraZeneca, another pharmaceutical company developing a vaccine, showed similar partial findings from its trials in the United Kingdom.
Dr. Patrick Moore, a virologist at the University of Pittsburgh who reviewed the Moderna vaccine as part of the U.S. Food and Drug Administration’s advisory committee, was optimistic about its prospects of controlling “viral shedding” but pushed the drugmaker to collect a third round of nasal swabs.
“What I’d like to see is at that third date when we know that people are protected, what is the rate of viral shedding among the general population?” Moore said in an interview. “At that second visit, they showed a reduction in shedding among the few people that they actually tested who were positive, so that’s a very, very good sign.”
How else can we determine this crucial knowledge?
It could be months before we know for sure, Reingold said. He suggested separate studies that look at how infections spread within households where at least one person had been vaccinated but noted the complexity and difficulty involved in such research.
At least one group of researchers, led by Dr. Larry Corey at the University of Washington, hopes to enroll college students in a large study, administer half a vaccine and half a placebo, and observe transmission in a natural environment.
While scientists have been humbled by this virus, Moore did point to one previous vaccination experience that underscores the importance of preventing transmission. It was not for a viral disease but for the bacteria that causes pneumonia.
That initial vaccine only prevented serious illness, not transmission. Later, it was improved upon to also guard against transmitting
the virus but only approved for use in infants. Despite a smaller segment of the population receiving the later vaccine, it saved more lives.
“What we can see is that with this pneumococcal vaccine, there’s clear evidence that if you both prevent someone from developing the disease, and also prevent them from transmitting the agent that causes the disease, then you can save many lives, not just one life,” Moore said. “And the same thing we hope will be true for these vaccines. I would just feel more comfortable if there was direct evidence for that.”
The federal CDC is in the process of determining who will be prioritized in the next round of vaccinations. As part of that, researchers developed a model that included the implications of a vaccine that prevents all transmission versus one that only stops disease.
The most deaths are averted in either scenario by
prioritizing those 65 and older over high-risk adults and essential workers. But when the vaccine also prevents transmission, twice as many lives are saved than when it only guards against disease.
“If the vaccine helps prevent transmission, to one extent or another, it suggests we could have a herd immunity effect,” Reingold said. “If it doesn’t prevent that, fundamentally, herd immunity is not going to pertain, and the real question will be vaccinating as many people as possible to protect them through direct vaccination.”
If these vaccines perform similarly to past ones, for measles and influenza and so on, then they should effectively mitigate viral shedding and, thus, transmission. But if this virus has taught us anything, Reingold said, “Frankly, it’s a crapshoot to try and extrapolate from different viruses and different vaccines to this one.”