Scientists continue to work on better ways to design, conduct and evaluate non-randomized (., observational) studies to assess how well flu vaccines work. CDC has been working with researchers at universities and hospitals since the 2003-2004 flu season to estimate how well flu vaccine works through observational studies using laboratory-confirmed flu as the outcome. These studies currently use a very accurate and sensitive laboratory test known as RT-PCR (reverse transcription polymerase chain reaction) to confirm medically-attended flu virus infections as a specific outcome. CDC’s studies are conducted in five sites across the United States to gather more representative data. To assess how well the vaccine works across different age groups, CDC’s studies of flu vaccine effects have included all people aged 6 months and older recommended for an annual flu vaccination. Similar studies are being done in Australia, Canada and Europe. More recently, CDC has set up a second network the Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN) that looks at how well flu vaccine protects against flu-related hospitalization among adults aged 18 and older.
Influenza vaccine production begins as early as 6 to 9 months before the beginning of vaccine distribution. Even with this early start, it isn’t possible to complete the entire production and distribution process prior to flu season, in part because of the limited number of influenza vaccine manufacturing plants in the United States and the large number of doses that are produced each year. Instead, influenza vaccine distribution takes place in a phased fashion over a number of months. Distribution usually begins in late summer and is complete near the end of November or early in December. This may result in some uncertainty making it difficult for vaccine providers to plan their vaccination activities. Manufacturers and distributors try to get some vaccine to as many providers as possible as early as possible so that they can begin vaccinating their patients.