As coronavirus vaccines are distributed throughout the country, a big question remains: Even though the vaccines reduce symptomatic COVID-19, can vaccinated individuals still transmit the virus?
The answer could significantly impact whether there is a fourth wave of coronavirus infections this spring, according to new models from the Fred Hutchinson Cancer Center. If the FDA-approved Pfizer and Moderna vaccines prevent transmission, that may have a big impact on virus spread.
“That would get us to herd immunity more quickly, prevent more cases and deaths, and prevent or severely limit a fourth wave of cases in the spring that would persist through the summer,” said Dr. Joshua Schiffer, an infectious disease researcher at Fred Hutch. “It ends up amounting to huge differences in cases and deaths, and whether we’ll have to deal with this one more time in the spring and summer.”
Schiffer worked on a recently-released preprint and its companion paper on this topic. While those findings haven’t been peer reviewed, outside observers such as Dr. James Wood, a mathematician and professor who specializes in vaccine-preventable disease, said the studies offer interesting ideas on how to figure out whether the vaccines prevent transmission — findings that could help us better predict the pandemic’s path in 2021.
“The main takeaway message is that despite the very good efficacy values in the Moderna, etc. studies, we are not yet sure of the impact of these vaccines on transmission — and that there could be big variations in community-level effects,” Wood said.
How vaccines work
Vaccines can prevent an exposed individual from developing symptoms of a disease. But whether that vaccine prevents an infection — and thereby stops the disease from spreading to others — presents a gap in the literature, Schiffer said.
There are some studies that offer optimism. For example, one study shows that if a vaccinated child gets the chickenpox, they’re far less likely to infect unvaccinated household members if exposed to the virus.
Similarly, other studies show that increases in childhood vaccinations for a bacterial infection lead to a reduction in cases and deaths among the unvaccinated elderly, indicating that vaccination also impacts community transmission.
“Those studies are very suggestive that vaccines protect against transmission, but they aren’t a slam dunk,” Schiffer said.
If a vaccine doesn’t completely stop transmission, it can still be effective in reducing community spread, but it takes more time.
“You would still march toward herd immunity, but you just march more slowly,” Schiffer said.
Modeling possibilities in Seattle
Hutch researchers modeled the impact of vaccines that would stop transmission versus ones that would not in Seattle’s King County, which has a population of 2.25 million people. They also looked at what might happen absent any vaccine.
Without a vaccine and without a change in current social distancing behavior, the county could be seeing up to 5,900 cases and 99 deaths per day between January and July of next year.
The numbers improve dramatically with a vaccine, and even more if that vaccine prevents transmission.
A vaccine that reduces a vaccinated person’s symptoms but doesn’t stop them from transmitting a possible infection would prevent roughly 40% of infections and 60% of deaths during the vaccine roll out period, the Hutch models show. If the vaccine has both effects, though, it could prevent 70% of infections and 70% of deaths.
Other factors can play a role, such as the speed and size of the rollout. Those become increasingly important if the vaccine does not block transmission.
“If we roll out more slowly, the differences become accentuated,” Schiffer said. “This really matters: the speed of the rollout is the strongest variable we see.”
How people behave is significant, too. Physical distancing, masking and remote work all play into the impact and the severity of the next wave.
The role of asymptomatic transmission — which is still debatable — could also influence the vaccine’s impact, said Dr. Natalie Dean, a University of Florida biostatistics professor who was not involved with the Fred Hutch study. Even if the vaccine converted all symptomatic infections to asymptomatic infections, there still could be some reduced transmission.
How can we find out?
One way to determine whether the COVID-19 vaccines also stop transmission is a Human Challenge study, one of the ideas described in the Hutch papers. That would mean taking young, healthy volunteers and injecting them with either the true vaccine or a placebo. Researchers would then intentionally expose them to the virus before checking their viral load, a measure of how likely someone is to pass on the virus.
These kinds of studies have been done previously for some respiratory viruses and malaria, Schiffer said. But there are some ethical concerns. People enrolled in that study could develop the disease themselves and risk their own wellbeing and would have to be followed very closely.
Another idea is to enroll young healthy people on a university campus, inject them with either the true vaccine or a placebo, then monitor them often to check for the presence of virus and whether they develop symptoms. Since college campuses have shown substantial COVID-19 transmission, it’s likely many will be exposed to the virus naturally. While the frequent testing would be a massive undertaking, the environment would capture a real-world situation.
“Either of these studies are potentially important and could complement each other,” Schiffer said.
Schiffer said he’s hopeful that studies measuring transmission among individuals who have received the Moderna and Pfizer vaccines will be rapidly conceived and performed. Those results would be critical for policy makers, informing them whether herd immunity has been reached in early spring, “which will in turn help predict whether it is safe to slowly reopen society,” Schiffer said.
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