Community circulation patterns of oral polio vaccine serotypes 1, 2, and 3 after Mexican national immunization weeks

Abstract

Background: With wild poliovirus nearing eradication, preventing circulating vaccine-derived poliovirus (cVDPV) by understanding oral polio vaccine (OPV) community circulation is increasingly important. Mexico, where OPV is given only during biannual national immunization weeks (NIWs) but where children receive inactivated polio vaccine (IPV) as part of their primary regimen, provides a natural setting to study OPV community circulation.

Methods: In total, 216 children and household contacts in Veracruz, Mexico, were enrolled, and monthly stool samples and questionnaires collected for 1 year; 2501 stool samples underwent RNA extraction, reverse transcription, and real-time polymerase chain reaction (PCR) to detect OPV serotypes 1, 2, and 3.

Results: OPV was detected up to 7 months after an NIW, but not at 8 months. In total, 35% of samples collected from children vaccinated the prior month, but only 4% of other samples, contained OPV. Although each serotype was detected in similar proportions among OPV strains shed as a result of direct vaccination, 87% of OPV acquired through community spread was serotype 2 (P < .0001).

Conclusions: Serotype 2 circulates longer and is transmitted more readily than serotypes 1 or 3 after NIWs in a Mexican community primarily vaccinated with IPV. This may be part of the reason why most isolated cVDPV has been serotype 2.

Keywords: Mexico; VDPV; inactivated polio vaccine; oral polio vaccine; polio.

Figure 1.

OPV circulation in Mexico following an NIW: enrolled children vs household contacts. This graph contains data for shedding after all 3 NIWs combined. The number of samples from enrolled children and household contacts respectively were as follows: 138 and 275 from 0.5 months, 138 and 276 from 1.5 months, 138 and 275 from 2.5 months, 71 and 141 from 3 months, 72 and 141 from 4 months, 71 and 142 from 5 months, 70 and 142 from 6 months, 69 and 138 from 7 months, and 68 and 136 from 8 months after an NIW. Abbreviations: NIW, National Immunization Weeks; OPV, oral polio vaccine.

Figure 2.

OPV circulation in Mexico following an NIW by serotype. This graph contains data on shedding of specific OPV serotypes from all 3 NIWs combined. Of note, 30 samples contained more than 1 serotype, so the sum of the frequency of the 3 serotypes sometimes exceeds the total percentage of stools containing OPV strains. Abbreviations: NIW, National Immunization Weeks; OPV, oral polio vaccine.

Figure 3.

Algorithm for determining whether OPV strains were acquired through direct vaccination vs community spread. Abbreviation: OPV, oral polio vaccine.

Figure 4.

Serotypes seen in positive samples by mode of acquisition. The 95% confidence intervals of each proportion were calculated using the modified Wald method. Of note, 30 samples contained multiple serotypes, so the percentages do not add up to 100%.