Population immunity to polioviruses among preschool children from four urban underserved low income communities, United States, 1997-2001.

Background: In 1997, the Advisory Committee for Immunization Practices (ACIP) recommended a change in polio vaccination policy, the first in 30 years, from the oral poliovirus vaccine (OPV) to a combined OPV/inactivated poliovirus vaccine (IPV) sequential schedule for routine childhood vaccination. To evaluate the impact of the change in polio vaccination schedule on population immunity, we conducted a seroprevalence survey among low income preschool children from selected urban areas.

Methods: A repeat cross-sectional serosurvey was conducted during 1997-2001. Children 19-35 months of age receiving well-child care were recruited from outpatient clinics of academic medical centers. Serum samples were obtained and tested for neutralizing antibodies to polioviruses types 1, 2 and 3. A standardized questionnaire was administered to the parents or guardians of enrolled children.

Results: Seroprevalence remained high and stable during the study period. Among children sampled in the last study year (initiating their vaccinations from August 1997 through September 2000), seroprevalence was >/=95% to poliovirus serotypes 1 and 2 and >/=94% to serotype 3. Overall coverage with >/=3 doses of polio vaccine was 82-95% across sites during this period. The proportion initiating their vaccination schedule with IPV increased from 2.6% in study year 1 (children born October 1994-January 1997) to 80% in study year 4 (children born October 1997-January 2000).

Conclusions: Children in these underserved low income communities are well-protected against the spread of polioviruses; the introduction of IPV did not adversely impact coverage or seroprevalence. Continued monitoring is needed to evaluate population immunity in the absence of OPV circulation.