Modelling The Health And Economic Impact Of The Influenza Vaccine In Kenya

Abstract

Background: There is substantial burden of seasonal influenza in sub-Saharan Africa. In Kenya, national rates of influenza-associated respiratory illness were considerable during the emergence of the 2009 influenza pandemic. However, since then there have been no published national estimates of influenza burden in the post-pandemic period. Furthermore, an economic evaluation of the benefits of influenza vaccination in Kenya has not been conducted. Objectives: We set out to update estimates of the national burden of hospitalized and non-hospitalized influenza-associated severe acute respiratory illness (SARI) during a post-pandemic period (2012-2014) and describe the incidence of disease by narrow age categories. In addition, we estimated the cost-effectiveness of seasonal influenza vaccination among children using a dynamic transmission model to inform policy discourse on implementation of a national influenza vaccination program in Kenya. Methods: To determine the burden of hospitalised and non-hospitalised influenza associated SARI, we used data from Siaya County Referral Hospital from 2012-2014 to estimate age-specific base rates of SARI. We extrapolated these base rates to other regions within the country by adjusting for regional risk factors for acute respiratory illness (ARI), regional health-care utilization for ARI and the proportion of influenza positive SARI cases in each region, so as to obtain region specific rates of influenza-associated hospitalized and non-hospitalized SARI. For the economic evaluation I fitted an age-stratified dynamic transmission model to surveillance data from patients with influenza-associated severe acute respiratory illness (SARI) at five county referral hospitals in Kenya from 2010-2018. Using a societal perspective, we developed a decision-tree cost-effectiveness model and estimated the incremental cost-effectiveness ratio (ICER) per disability-adjusted life year (DALY) averted for three vaccine target groups: children 6-23 months (strategy I), 2-5 years (strategy II) and 6-14 years (strategy III) with either the Southern Hemisphere influenza vaccine (Strategy A) or Northern Hemisphere vaccine (Strategy B) or both (Strategy C: twice-yearly vaccination campaigns, or Strategy D: year round vaccination campaigns). To compare strategies we calculated the incremental net monetary benefits (INMB) xiv

using a willingness-to-pay (WTP) threshold of 1-51% of the annual gross domestic product per capita ($17-872). Results: For the period 2012-2014, the mean annual rate of hospitalized influenza-associated SARI among all ages was 21 (95% CI 19-23) per 100,000 persons, while rates of non-hospitalized influenza-associated SARI were approximately 4 times higher at 82 (95% CI 74– 90) per 100,000 persons. Mean annual rates of influenza-associated SARI were highest in children <2 years of age with annual hospitalization rates of 147 (95% CI of 134-160) per 100,000 persons and non-hospitalization rates of 469 (95% CI 426-517) per 100,000 persons. From 2012-2014, there were 8,929 (95% CI 8,153-9,751) cases of hospitalized influenza-associated SARI and 35,041 (95% CI 31,785-38,546) cases of non-hospitalized influenza-associated SARI per year. Using the dynamic transmission model we estimated the mean number of total infections to be 2-15 million per year. When vaccination was well timed to influenza activity, the annual mean ICER per DALY averted for vaccinating children 6-23 months ranged between $749-1,385 for strategy IA, $442-1,877 for strategy IB, $678-4,106 for strategy IC and $1,147-7,933 for strategy ID. For children 2-5 years it ranged between $945-1,573 for strategy IIA, $563-1,869 for strategy IIB, $662-4,085 for strategy IIC, and $1,169-7,897 for strategy IID. For children 6-14 years it ranged between $923-3,116 for strategy IIIA, $1,005-2,223 for strategy IIIB, $883-4,727 for strategy IIIC and $1,467-6,813 for strategy IIID. Overall, no vaccination strategy was cost-effective at the minimum ($17) and median ($445) WTP thresholds. Vaccinating children 6-23 months once a year had the highest mean INMB value at $872 (WTP threshold upper limit), however this strategy had very low probability of highest net benefit. Conclusions: Influenza virus was associated with substantial disease burden in Kenya, especially among very young children, <2 years of age. Vaccinating children 6-23 months once a year was the most favourable vaccination option, however, the strategy is unlikely to be cost-effective given the current WTP thresholds.1