Quantifying the Impact of Expanded Age Group Campaigns for Polio Eradication

Editor's note: While this article does not focus on communication, it does point to the impact of expanding age groups for vaccination campaigns making only limited contributions to increasing immunity levels. There are obvious communication considerations and risks associated with varying the age groups (vaccinating in different venues such as schools, answering caregiver questions as to why the change and what the impacts on their children may be, and dealing with heightened concerns around questions about fertility and other potential health consequences - to name a few). Because of the communication risks associated with such changes, we felt it was important to provide some information on the epidemiological benefits.

"[W]hat is the role of older children in transmission in areas where polio is currently endemically circulating and what benefit would vaccinating older children have in stopping endemic transmission?"

The delivery of oral polio vaccine (OPV) to children less than 5 years of age is standard practice in most supplemental immunisation activity (SIA) campaigns. This age group bears a majority of the morbidity and mortality of polio. However, recent programme strategy discussions have asked whether expansion of the OPV age group beyond 5 years would increase the impact of eradication efforts. Due to the higher costs of large-scale implementation of expanded age group (EAG) campaigns, these researchers sought to quantify the reduction in virus transmission, the effect of improved coverage, and the additional benefit of passive immunisation of such a strategy adjustment.

To do so, the researchers constructed a mathematical model of polio immunity and transmission calibrated to Zaria, Nigeria seroprevalence survey data. The model is individual-based, with transmission of poliovirus occurring through virus shedding and acquisition among individuals and their communities. For each individual, key aspects related to the research question are included, such as dynamics of the immune response to OPV , the relationship of mucosal immunity to probability of infection and subsequent virus shedding quantity, transmission of wild poliovirus (WPV) and vaccines, waning of mucosal and humoral immunity, and accessibility in campaigns. (When exposed to the virus, the probability of becoming infected is a function of the infectious units of virus and mucosal immune state.)

Even under the most optimistic assumptions that EAG campaigns (including 5- to 14-year-old children) would be employed in 6 SIAs per year and result in a 20% relative increase in overall coverage levels, the probability of elimination increased by only 16%. For 3 EAG campaigns, this increase was only 8% (from 24% at baseline to 32%). In contrast, adding 2 outreach campaigns per year with coverage in the hard-to-reach group at only half the baseline coverage of the easy-to-access group resulted in a 55% increase probability of elimination from the baseline. Furthermore, the amount of additional vaccine required in the targeted scenario was less than 1/10th and 1/5th of that for the largest and smallest EAG scenarios, respectively.

In contrast, in the absence of recent WPV circulation, immunity gaps may persist in older age groups due to low historical vaccine coverage. In this case, SIA campaigns with an older intended age group could be useful to prevent outbreak and accelerate local elimination efforts.

To further illustrate both the individual and combined impact of WPV transmission and EAG campaigns on immunity gaps, the researchers performed simulations on a hypothetical country. They analysed age-dependent immunity both with and without EAG campaigns and WPV circulation. For both endemic and previously polio-free conditions, they compared the fraction of seropositive older children (aged 5-14) with baseline, in the absence of EAG campaigns. Expanding the age group to reach children under 15 years of age was effective at filling immunity gaps in the absence of previous WPV circulation. This was true even under the conditions of low historical vaccination coverage and quickly waning mucosal immunity. In addition, under endemic conditions, age gaps in immunity resulting from previous low vaccination disappeared. In this case, EAG campaigns minimally increased the already high immunity. This result agrees with the detailed calibration results showing limited impact of EAG campaigns in polio-endemic regions and is consistent with the older ages (over 5 years) of observed polio cases from outbreaks in countries with low vaccination coverage and no recent WPV circulation.

In summary, if EAG campaigns can increase coverage among children aged less than 5 years, it becomes important to determine whether this increase would be in groups that have historically been largely unreached by vaccination campaigns. The results show that in endemic areas, reaching chronically missed children is more relevant to achieving eradication than reaching high coverage in older children. "As we move closer to polio eradication understanding these heterogeneities in immunity and how to optimally allocate available resources will be critical in realizing our endgame."