Adaptive vaccination strategies to mitigate pandemic influenza: Mexico as a case study

Abstract

Background: We explore vaccination strategies against pandemic influenza in Mexico using an age-structured transmission model calibrated against local epidemiological data from the Spring 2009 A(H1N1) pandemic.

Methods and findings: In the context of limited vaccine supplies, we evaluate age-targeted allocation strategies that either prioritize youngest children and persons over 65 years of age, as for seasonal influenza, or adaptively prioritize age groups based on the age patterns of hospitalization and death monitored in real-time during the early stages of the pandemic. Overall the adaptive vaccination strategy outperformed the seasonal influenza vaccination allocation strategy for a wide range of disease and vaccine coverage parameters.

Conclusions: This modeling approach could inform policies for Mexico and other countries with similar demographic features and vaccine resources issues, with regard to the mitigation of the S-OIV pandemic. We also discuss logistical issues associated with the implementation of adaptive vaccination strategies in the context of past and future influenza pandemics.

Figure 1. Flow chart of the stage progression of the individuals among the different epidemiological classes.

Figure 2. Age-specific hospitalization and case fatality rates given hospitalization estimated from cumulative morbidity and mortality data from the National Surveillance System stratified by four broad age groups at two time points into the S-OIV outbreak in Mexico.

The Ministry of Health issued an epidemiologic alert on April 16 and 17 and selective reporting of severe pneumonia cases ceased on April 29. Case fatality rate is estimated as the proportion of deaths among hospitalized pneumonia cases.

Figure 3. Age-specific incidence curves of clinical cases, hospitalizations and deaths in the context of the 2009 S-OIV outbreak in Mexico.

We considered a baseline situation with R₀ = 1.6 where no vaccine is used (top panel), a seasonal vaccination strategy (middle panel) where priority groups for vaccination are young children and seniors, as for seasonal influenza, and an adaptive vaccination strategy (bottom panel) where vaccine is allocated according to data on age-specific rates of mortality. The vaccination coverage is 20% and vaccination is initiated 25 days after the epidemic onset when an epidemiologic alert was issued in Mexico.

Figure 4. Comparison of concurrent vaccination strategies for novel S-OIV pandemic influenza scenarios.

Strategies include adaptive vaccination targeted at high-risk groups identified from real-time hospitalization and death data, and seasonal vaccination strategy targeted at young children and seniors. Plots illustrate the additional reduction (%, see color bar) in hospitalizations (top) and deaths (bottom) averted by the adaptive vaccination allocation strategy, compared to the seasonal influenza strategy, as a function of R (using plausible ranges, R = 1.4–1.8) and vaccination coverage (5–20%). In the left panels, vaccination is initiated 25 days into the outbreak, on April-17-2009, when the epidemiologic alert was issued. In the right panels, vaccination is initiated 37 days into the outbreak, on Apr-29-2009, when selective reporting of severe pneumonia cases ceased. The adaptive strategy allocates vaccine given the age-specific patterns of hospitalization and mortality available on those dates (Figure 2).