The Advisory Committee on Immunization Practices (ACIP) recommended initial target groups for vaccination against pH1N1 influenza, which include pregnant women, household contacts of infants younger than 6 months, health care and emergency medical personnel, persons aged 6 months through 24 years, and persons aged 25 through 64 years at higher risk for influenza-related complications.
Using the assumptions from the primary analysis, we find that the cost-effectiveness of vaccinating children and high-risk working-age adults against pH1N1 is within the range of cost-effectiveness for other vaccines recently recommended by ACIP, including seasonal influenza vaccine
, pneumococcal conjugate vaccine
, and HPV
. We also find that pH1N1 vaccination is cost-saving for high-risk individuals less than 65 years under a wide range of assumptions.
We did not include separate analyses for some of the initial target groups for recommendation, such as health care workers, pregnant women, or household contacts of infants younger than 6 months. However, health care workers and pregnant women would be included as part of the overall high-risk and low-risk group calculations. Given increased exposure of health care workers influenza and increased risk of complications of pH1N1 in pregnant women
, cost-effectiveness ratios would likely be at least as favorable as for corresponding target groups as defined by age and risk category.
The live attenuated formulation is not explicitly included in the current analysis. Live attenuated vaccine for seasonal influenza may be more effective than inactivated vaccine for young children
, yet recent data suggest that inactivated vaccines may be more effective for young adults
. However, there are no data on the effectiveness of live attenuated pH1N1 vaccine by age group and it is possible that this may differ from that for seasonal vaccine.
Emerging data on the epidemiology of pH1N1 influenza virus infection were used in the simulation model where available, but some assumptions were based on data from seasonal influenza. These include the probability that an individual will seek medical attention if they experience influenza-like illness. If individuals are more likely to seek medical attention if they think they have pH1N1 infection, these results represent a conservative approach to assessing the cost-effectiveness of vaccination. Similarly, if the costs of treating pH1N1 infection are substantially higher than for seasonal influenza, the results of this analysis will also be conservative. We did not consider any costs related to potential school closures or mandated absences from school or work due to illness. If these costs were appreciable, the results would be more favorable for vaccination if these additional costs of illness were included.
This analysis differs from dynamic models, which model the indirect effects of vaccination. One such model suggested that vaccinating school-aged children and adults between the ages of 30 and 40 would be most cost-effective.
Dynamic models simulate the transmission of infection among individuals and estimate the reduction in infections as a result of reduced transmission among unvaccinated age groups (e.g., vaccinating school children will reduce transmission to individuals of other ages and result in indirect effects of reducing illness and deaths in infants and the elderly). The current analysis intentionally excludes possible indirect effects of vaccination and restricts the primary analysis to the costs and health benefits to the vaccinated individual. Required coverage rates to generate herd effects for pH1N1 are unknown and substantial uncertainty exists for the required minimum coverage level for seasonal influenza
. Even if higher coverage rates are attained or pH1N1 vaccination compared to seasonal influenza vaccination, the effect of herd immunity is uncertain.
Another rationale for restricting the analysis to individual-level benefits relates to societal preferences. A policy of vaccinating school children to prevent illness in other age groups assumes that public preferences are consistent with trading off the health and well-being of school-aged children (in the form of risk for vaccination-related adverse events) to protect people in other age groups. Evidence suggesting that societal preferences may be more consistent with prioritizing child health over adult health could clearly support the vaccination of children if the expected benefits to the child outweighed the potential costs and risks but the decision to vaccinate a child may only consider benefits and risks to the vaccinated child.
Given that the inclusion of herd effects would result in an increase in the health benefits associated with vaccination, an extension of the current analysis to include herd effects would result in more favorable cost-effectiveness results.
The costs of vaccination have a substantial impact on cost-effectiveness results. Uncertainty exists as to the proportion of individuals likely to get vaccinated in each type of setting. Since costs of vaccination will vary with setting, these are key assumptions for the analysis. For higher attack rate scenarios, cost-effectiveness ratios vary about 10% across settings, however, if most people are vaccinated in the physician office setting and require a vaccine-specific visit, vaccination costs will be higher and associated cost-effectiveness ratios will be less favorable. On the other hand, if only one dose is required for vaccinating children, the associated costs will be lower and cost-effectiveness ratios will be more favorable. Recent data from the 2009–2010 pH1N1 vaccination season indicate that a mix of settings was used for vaccination against pH1N1 influenza.
Results of sensitivity analysis varying the costs of vaccination could provide useful information for future decision making given the sensitivity of the results to this input parameter.
Initiation of vaccination after the start of the season will affect the cost-effectiveness of vaccination depending on the timing of availability relative to the start, duration, and intensity of influenza activity in a community. For children who require two doses, vaccination may not be cost-effective if vaccine is delivered such that full protection is not achieved until after the 8th week (the peak) of a hypothetical influenza season. For adults and children requiring only one dose, results are similar but the timing of vaccination required will differ since only one dose is assumed to be required for full protection. Cost-effectiveness results would differ if additional pandemic waves caused by a similar virus were to occur within the same vaccination year, or if vaccination later during a single pandemic wave provided some beneficial immunologic priming for subsequent vaccination against a drifted influenza virus, or if the pattern of disease during the pandemic wave does not conform to our model of a hypothetical season.
Comparison to Other Economic Studies of pH1N1 Vaccination
Estimates of the economic impact of vaccination are available for the US and other countries. In the US, Beigi et al. evaluate the economic value of vaccinating pregnant women, a very high risk group not included in our analysis, and report favorable cost-effectiveness ratios for maternal influenza vaccination.
Khazeni et al. (2009) evaluate the cost-effectiveness of vaccination against pandemic influenza A (H1N1) for a major US metropolitan city and early vaccination to be cost-saving.
Lee et al. (2010) estimate averted lost productivity costs for an employee population but do not report results using an economic metric such as an incremental cost-effectiveness ratio making it difficult to compare these results with those from our study.
Sander et al (2010) find the cost-effectiveness of a mass immunization program for pandemic H1N1 to be favorable but this study does not evaluate the cost-effectiveness of individual age and risk groups.
Baguelin et al. (2010) use a dynamic model to evaluate vaccination against pandemic influenza A(H1N1) in England.
Our analysis complements other published analyses both for the US and abroad by evaluating the cost-effectiveness of individual age and risk groups and considering explicitly the effects of delay in vaccination on cost-effectiveness. While it is difficult to directly compare cost-effectiveness analyses across countries due to differences in costs of services and level of intensity of care, results from other countries were consistent with ours in estimating favorable cost-effectiveness for mass vaccination in England and Canada. Our analysis provides additional information to the previously published studies by providing incremental cost-effectiveness ratios for separate age and risk groups relevant to the US setting. In addition, we explored sensitivity analyses relevant to the US decision maker perspective for costs of vaccination, influenza illness rates, and delays in vaccination using the best available information at the time of the pandemic.
None of the published studies reviewed above appear to have accounted for costs or health consequences potentially associated with H1N1 vaccination, except for the Khazeni et al and Beigi et al. studies. The exclusion of vaccination-related adverse events would yield more favorable results for vaccination compared with a more comprehensive analysis that included adverse events. Both costs and health effects of potential vaccine adverse events are explicitly accounted for in our analysis. Additionally, outside the US, adjuvanted vaccine was typically used and this formulation could result in a different risk profile than non-adjuvanted vaccine. The study by Khazeni et al. assumed the use of adjuvanted vaccine for a US setting and is not directly comparable to our study due to assumed differences in effectiveness and side effect profile between the two formulations.
Vaccination for pH1N1 influenza for children and young adults is cost-effective compared to other preventive health interventions under a wide range of scenarios. Delayed availability of pH1N1 vaccine results in less favorable cost-effectiveness results. A vaccination program for pH1N1 influenza for target groups can be justified from an economic perspective when indirect benefits are not considered and assuming that vaccine supplies are sufficient. Additional economic and health benefits beyond direct benefits would only add to the cost-effectiveness of pandemic influenza vaccination.