2012-13 Influenza vaccine effectiveness: a preliminary estimate
I found an update from the Centers for Disease Control and Prevention regarding the effectiveness of the trivalent seasonal vaccine towards this year’s influenza outbreak. Last week I presented an update of the scope of the seasonal flu outbreak in Pennsylvania, and it does not yet show any signs of beginning to diminish. Indeed, local doctors’ offices don’t even want to see patients for influenza, unless fever is over 105°C, or if fever has persisted for longer than 5 days, due to the high number of cases and the potential for a patient to transmit the virus to another patient in the office.
The CDC’s “Early estimates of seasonal influenza vaccine effectiveness” presents patient data collected from Washington, Wisconsin, Michigan, Pennsylvania, and Texas. Over 1000 patients reporting acute respiratory illness between December 3, 2012 and January 2, 2013 were enrolled in the study. Nasal and throat swabs were taken from the enrollees, and their vaccination status was also reported. Specimens were then tested for influenza virus by polymerase chain reaction. Of the 1155 patients enrolled, 236 (20%) tested positive for influenza A ( all were H3N2, the variety in the trivalent seasonal vaccine), and 180 (16%) tested positive for influenza B. The remaining 739 cases (64%) were negative for influenza.
Because the vaccination status was known for all of these patients, the effectiveness of the vaccine can be determined. Of the 416 patients who were tested positive for either influenza A or B, 133 of them (32%) reported receiving the vaccine. In the influenza-negative patients, 411 out of 739 (56%) reported receiving the vaccine. These figures are used to calculate a vaccine effectiveness (VE) for both subtypes of approximately 62%, a figure which is very much in line with data from previous years when there is good matching of vaccine components to circulating subtypes of virus. An editorial note by the CDC in the report states that although this isn’t perfect protection, it does represent the best mechanism of currently preventing influenza. To date, there have been 40 flu-related deaths in Pennsylvania alone, out of 16,511 cases. The CDC further points out that new vaccine technologies are necessary. A new technology to produce seasonal influenza vaccines in insect cell lines might provide a way to make more effective vaccines.