Modeling Poliovirus Transmission and Responses in New York State

Abstract

Background: In July 2022, New York State (NYS) reported a case of paralytic polio in an unvaccinated young adult, and subsequent wastewater surveillance confirmed sustained local transmission of type 2 vaccine-derived poliovirus (VDPV2) in NYS with genetic linkage to the paralyzed patient.

Methods: We adapted an established poliovirus transmission and oral poliovirus vaccine evolution model to characterize dynamics of poliovirus transmission in NYS, including consideration of the immunization activities performed as part of the declared state of emergency.

Results: Despite sustained transmission of imported VDPV2 in NYS involving potentially thousands of individuals (depending on seasonality, population structure, and mixing assumptions) in 2022, the expected number of additional paralytic cases in years 2023 and beyond is small (less than 0.5). However, continued transmission and/or reintroduction of poliovirus into NYS and other populations remains a possible risk in communities that do not achieve and maintain high immunization coverage.

Conclusions: In countries such as the United States that use only inactivated poliovirus vaccine, even with high average immunization coverage, imported polioviruses may circulate and pose a small but nonzero risk of causing paralysis in nonimmune individuals.

Keywords: New York; immunization; modeling; outbreak; polio.

Figure 1.

A, New York State (NYS) wastewater surveillance results for outbreak counties, as well as the nonoutbreak counties that reported wastewater surveillance results. Each colored box shows the surveillance outcome for the corresponding NYS counties, indicating weeks with at least 1 positive sample result (red and purple boxes), only negative or indeterminate sample results (green boxes), or no wastewater surveillance results (light gray). The index patient presented with paralysis onset in June 2022 (week 25), and the case was reported in July 2022 (week 29), with designation of weeks of the year based on the convention used by the Morbidity and Mortality Weekly Report (MMWR) corresponding to a 52-week year. The vertical black line between weeks 52 and 1 represents 1 January 2023. Each county may include more than 1 sewer shed and multiple sampling sites, for which we aggregate at the county level. For Kings County, the 1 sewer shed that was repeatedly positive included a small portion of Queens, which we attributed to Kings County only. Not specifically shown are three 24-hour composite specimens and 14 large-volume specimens collected to increase sensitivity of wastewater surveillance in critical times and/or regions. Nonoutbreak counties that have not been subject to wastewater surveillance are not listed; see county map in (B). B, Map of outbreak and nonoutbreak counties with and without wastewater surveillance. C, Variability in reported polio immunization coverage by county based on the best available, different metrics reported by the New York City Department of Health and Mental Hygiene and the New York State Department of Health at the beginning of the outbreak [10, 19].

Figure 2.

Monthly new infections by prior immunity state under different mixing assumptions. A–C, For each mixing assumption, new infections in the undervaccinated population of the outbreak counties are shown on the left, and the total number of expected new infections for New York State on the right. Text insets in each panel show the corresponding expected paralytic cases (derived from Table 1) and the total expected infections (area under the solid curve). See Supplementary Figure 1 for the bounding scenarios of complete isolation and homogeneous mixing. Abbreviations: IPV, inactivated poliovirus vaccine; LPV, live poliovirus; OPV, oral poliovirus vaccine.

Figure 3.

A, Effective immune proportion (EIP) for the outbreak counties undervaccinated subpopulation for 3 mixing assumptions showing the decline in population immunity starting with the shift in routine immunizations to inactivated poliovirus vaccine/oral poliovirus vaccine (IPV/OPV) (1997) and then to IPV-only (2000). EIP* shows the threshold below which the population can sustain imported transmissions, with oscillation in the threshold for potential die out (EIP*) reflecting seasonality. The transient increase in EIP in 2022 reflects the population immunity gained by transmission of the outbreak virus in New York State (NYS). Population immunity will continue to fall in the future, implying ongoing seasonal risk of future outbreaks in NYS. B, Net reproduction number (R_n) for NYS for 3 mixing assumptions between 2022 and 2025 highlights the risk of sustained transmission of imported type 2 polioviruses when R_n >1, as occurred with the index patient presenting in June of 2022.