Immunogenicity of Combined Bivalent OPV and IPV Vaccines at 9 - 12 Months of Age Compared to bOPV Alone in Malnourished and Non-Malnourished Pakistani Infants.

The development of chronic malnutrition is a complex interplay of multiple factors; including genetic predisposition and a whole host of environmental insults. In the urban squatter and peri-urban settlement environments of Karachi where the Aga Khan University (AKU) has established surveillance programs, half of all infants have moderate to severe chronic malnutrition (height for age Z score less than -2 SD) by the time they are 9 -12 months old. IPV has the advantage of parenteral route, thereby bypassing the damaged gut mucosa of malnourished children. It also does not cause the rare vaccine-associated paralysis. Its effectiveness depends on stimulation of serum (blood) neutralizing antibodies that block the spread of poliovirus to the central nervous system. The main disadvantages that have precluded IPV use in low-income countries is high cost, difficulty in adding another injectable vaccine in the EPI schedule at 6, 10, and 14 weeks, and lack of ability to induce a strong mucosal immune response and therefore prevent enteral infection. Additionally, maternal antibodies at this early age neutralize IPV quite effectively. However, as the median age of polio in Pakistan is 15 months for poliovirus type 1 and 18 months for poliovirus type 3, it may be feasible to give a single IPV dose at an older age, avoiding the effects of maternal antibodies and boosting immunity against polio before the majority of these children enter their risk period in Pakistan. Enhanced potency IPV (eIPV) with higher antigen content yields greater than 90% seropositivity against all 3 types after one dose and 100% seropositivity after two doses. Thus using eIPV combined with OPV at 9 - 12 months in moderate to severe chronically malnourished infants may provide improved seroconversion as well as some stimulation of the mucosal immune response. Our proposed idea is to evaluate if IPV antigen given later in life may act together to boost humoral and mucosal immunity in children belonging to low-income background in Karachi who have moderate to severe chronic malnutrition (height for age Z score less than -2SD). We also intend to compare eIPV + OPV with OPV only in non-malnourished infants at 9 -12 month of age. Thus, the proposed study is a combination of two trials, with study population stratified by nutritional status, each with a reference arm (bOPV) and an experimental arm (bOPV plus IPV).