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Math Problem: What's The Best Strategy For COVID-19 Vaccination?

Older adults and first responders wait in line to receive a COVID-19 vaccine late last month at the Lakes Regional Library in Fort Myers, Fla.

Only a vaccine will save America from the COVID -19 pandemic. At least that's the opinion of nearly all public health officials.

Obviously, vaccine manufacturers are critical to any vaccine campaign. But there's another group that plays a less obvious but still crucial role in making sure vaccines do what they're intended: mathematicians.

Even if the Biden administration releases all available doses of the two authorized COVID-19 vaccines, for a while at least, supplies will remain limited. How best to use that limited supply is a question mathematicians can help answer.

"Mathematical models are very, very useful to guide policy," says Laura Matrajt, a builder of mathematical models at the Fred Hutchinson Cancer Research Center in Seattle. They can help with decisions about who gets the vaccine first when supplies are limited.

"There are a few factors that really shift how the prioritization would go in a strong way," says Daniel Larremore, a computer scientist at the University of Colorado BioFrontiers Institute. "One of those is how much is the virus spreading as the vaccine is being rolled out? And another factor is. How fast is the vaccine being rolled out?"

It's also important to know how effective a vaccine is at preventing disease, how long protection lasts, and whether it not only prevents someone from getting sick but also from transmitting COVID-19. Unfortunately, only the efficacy of Pfizer-BioNTech and Moderna vaccines is known, not the other two variables.

Even in the face of missing information, modelers can provide some guidance. Larremore says to end a pandemic, it generally makes sense to vaccinate those most capable of spreading disease.

"However," he says, "if the vaccine is rolled out slowly, which is currently the case, and if community transmission is high, which is also currently the case, then it becomes better to prioritize adults over 60 to minimize mortality."

This is close to what the Centers for Disease Control and Prevention is currently recommending, although the CDC guidelines also call for vaccinating people likely to be exposed to the virus, such as front-line health care workers.

But even if a mathematical model suggests the most effective path, it doesn't provide all the answers public health officials need.

"You still have to make some difficult value decisions," says Mark Jit at the London School of Hygiene and Tropical Medicine. "Like how many lives in older people are you going to save, how many lives in younger people are you going to save, how many lives in different people, richer, poorer people. So there are some tricky value decisions, but those are inherent in the fact that we have limited supplies of the vaccine, so we have to make hard decisions about who to give it to first."

Right now, modelers are trying to help public health officials decide if it makes sense to use a single dose of the Moderna and Pfizer vaccine to extend the limited supply, even though the vaccine has only really been tested using a two-dose regimen.

Matrajt of the Fred Hutchinson Cancer Research Center has a model that allows her to test various "what if scenarios."

"If the first dose is highly efficacious, and we have stringent social distancing interventions in place, then vaccinating with a single dose is optimal," Matrajt says, but the strategy only works if a single dose is efficacious.

"We should do whatever it takes to know what the efficacy of a single dose is," she says.

What's clear from all the models is that rolling out the vaccine as quickly as possible is essential.

"If you don't roll out the vaccine fast enough, the effect of the vaccine will be very, very limited," Matrajt says.

Gerardo Chowell, a mathematical epidemiologist at Georgia State University, puts it even more bluntly. "Delays in vaccination, delays in testing, are costing us lives."
