The close correlation between estimated pandemic influenza cases and filled prescriptions () can be used as evidence that antiviral drugs were mostly used to treat those who were clinically ill (i.e., recommendations regarding use were essentially followed). Restricting the use of antiviral drugs to treating the clinically ill meant that preventing clinical cases from deteriorating into severe cases requiring hospitalizations was likely to have been among the major effects of antiviral drug use. By our estimates, this strategy worked; ≈8,000–13,000 hospitalizations were averted (). This reduction is equivalent to ≈4–5% of the total estimated pandemic (H1N1) 2009–related hospitalizations (1
We found no other studies with which to compare our methods and results. We compared the accuracy of the IMS database using unpublished data from the Behavioral Risk Factor Surveillance System (BRFSS), conducted in 49 states (excluding Vermont, the District of Columbia, and Puerto Rico). From September 1, 2009, through March 31, 2010, adults (>18 years old) responding to the BRFSS telephone survey were asked whether they had influenza-like illness (ILI) (defined as having had a fever with cough or sore throat) in the month preceding the interview. They were also asked if they sought medical care for their ILI condition and if they were prescribed antiviral drugs to treat their illnesses. Extrapolating the results to the national level in the period covered by the survey, we found that ≈54 million adults reported having ILI symptoms. Of those who reported having ILI and sought medical care, 4.1 million adults reported they were prescribed influenza antiviral drugs (oseltamivir or zanamivir) during August 2009–March 2010. The IMS database recorded 6.86 million prescriptions in the same period (); ≈40% for those 0–17 years of age (), leaving ≈4.1 million filled prescriptions for adults. This estimate is close to the number recorded by the BRFSS survey and further supports the idea that few prescriptions were for prophylaxis or personal stockpiles.
There are many limitations to this study; the biggest is the uncertainty regarding the effectiveness of the drugs in preventing hospitalizations. The effectiveness of the drugs in reducing risk for hospitalization caused by pandemic (H1N1) 2009 may vary considerably from estimates reported for nonpandemic strains of influenza virus. The data are also limited in that we cannot verify if those persons who filled a prescription were actually clinically ill from pandemic (H1N1) 2009 or to what extent they adhered to the drug regimen. We addressed this issue by allowing a wide range in drug effectiveness and a relatively large percentage of prescriptions filled for conditions other than direct treatment of pandemic (H1N1) 2009.
We were unable, because the available literature did not contain sufficiently reliable estimates of effectiveness of antiviral drugs against death, to estimate the number of deaths averted by treatment with antiviral drugs. Shrestha et al. (1
) estimated that deaths caused by pandemic (H1N1) 2009 were equivalent to 1.5% of children’s hospitalizations and 6% of hospitalizations for persons of all other ages. Assuming that hospitalizations averted generate similar percentages of deaths averted, then the use of antiviral drugs prevented 27–40 deaths in children 0–17 years of age and 395–597 deaths in adults of all ages (using median values of hospitalizations averted; ).
If during the next pandemic there is a desire to produce better quality estimates (perhaps even produce estimates at regular intervals during the event), then additional data collection systems must be developed to overcome some of these limitations. For example, measuring the number of prescriptions filled for prophylaxis or personal stockpiles or degree of adherence can only reliably be conducted by interviewing patients and physicians. Improving estimates of impact of filled prescriptions in reducing adverse health outcomes during an event will require a large case–control study. Policy makers will have to determine if the value of such information warrants the investment in such data collection systems.
Our results also highlight how the use of influenza antiviral drugs during a pandemic is likely to be beneficial, notably through a presumed reduction in the demand for hospital-based resources. Reduced demand will also reduce costs of hospitalizations. Assuming a cost per influenza-related hospitalization of US$5,000–$7,000 per patient admitted (adjusted to 2009 dollars) (22
), averted hospitalizations saved ≈$42 million to $88 million (based on median values of hospitalizations averted; ). A detailed cost-effectiveness analysis, including an in-depth consideration of the costs of hospitalizing pandemic (H1N1) 2009 patients, is the subject of a separate analysis.
If the next influenza pandemic causes greater numbers of severe cases and hospitalizations than in 2009, there may be an increased demand for antiviral drugs for treatment and prophylaxis. Such increased demand could overwhelm the existing commercial distribution chains. Therefore, public health officials should consider these estimates as an indication of success of treating patients during the 2009 pandemic and a warning for the need for renewed planning to cope with the next pandemic.