When is the Best Time to Administer Polio Vaccines?

Micaela Martinez, PhD, postdoctoral fellow in the Department of Ecology and Evolutionary Biology at Princeton University, explains how understanding the reproductive ratio of polio can help timing vaccine campaigns.

Interview Transcript (slightly modified for readability)

“The reproductive ratio, depending on the context we can refer to this as the basic reproductive number, is [the number of] susceptible individuals [an infected person can] pass on that infection to. So if I was infected and I walked into this room and I gave a pathogen to three different people, then the reproductive ratio would be three, because I am one person and I have infected three.

The work that I’ve done on polio historically in the United States has shown that this reproductive ratio can change by an order of magnitude depending on what time of year it is. You can have very low reproductive ratios, even below one in some places, during the winter-time, and then extremely high, like values over 20 [or] over 50, for a very, very short period of time during the summer.

The reason that this number is important is because the reproductive ratio tells you about how transmissible an infection, [which will tell] you how much vaccine you need to launch in a population in order to keep that pathogen out. So for a disease like measles, which [has] very high transmission [rates], we have to vaccinate a lot of kids to keep measles out of the population; typically, over 95% of individuals need to be vaccinated. For other [less-transmissable] infectious diseases, because they’re less transmissible you don’t need to immunize as many individuals.

Rolling this back to polio, if you have a time of the year [when] the reproductive ratio is very low, and then you have a time of the year when the reproductive ratio is very high, then that changes the number of individuals that you would have to immunize if you were to try to get rid of that disease during one part of the year versus the other.

The reason this is extremely important right now for polio is that we have two different vaccines against polio: the inactivated vaccine (IPV) and the oral polio vaccine (OPV). These vaccines are extremely different in the way they work. One of them, the oral polio vaccine, which has been used for the eradication initiative, stimulates immunity in the gut where the virus replicates. We know that this vaccine is extremely good at protecting individuals from infection, but the other vaccine, IPV, is an injectable vaccine and it stimulates antibodies in the blood. It’s a completely open question right now [of] whether or not this vaccine [fully] protects people from infection, or if it just reduces how much they transmit, or if it just protects them from paralysis. We don’t know, but right now we’re beginning globally the transition from OPV, the oral polio vaccines that’s very effective, to IPV, the vaccine where we have some big question marks.

It’s important to know about these reproductive ratios for polio and how those change throughout the seasons because as we move to this new vaccine, we are going to be launching that vaccine in many countries that haven’t been using it, with some big unknowns; [therefore] we need to measure the efficacy of that vaccine and then we need to measure the transmissibility of polio in these countries that are going to be using it, to figure out if we could even drive polio out of populations with that vaccine, because we don’t know.

That’s why these numbers become really, really important, because we have to come up with targets for how many kids to immunize and we need to know locally the reproductive ratio and how that changes from one time of year to the other.”