Updated and age-adjusted estimates of influenza vaccine effectiveness for the 2012-13 season show it was moderately effective overall, but much less so in seniors.
Overall, influenza vaccination reduced the risk for medical visits resulting from influenza A and B by 56% — including by 47% from influenza A (H3N2) and by 67% from influenza B, according to a report issued by the Centers for Disease Control and Prevention.
The preventive benefits against influenza B were consistent across age groups. The adjusted vaccine effectiveness estimates against influenza A (H3N2) viruses also were largely consistent (46%-58%) for people ages 6 months to 64 years, but the vaccine effectiveness estimate was only 9% among people ages 65 and older.
Confirmation of the protective benefits of the 2012-13 influenza vaccine among people 64 and younger, the report’s authors wrote, “offers further support for the public health benefit of annual seasonal influenza vaccination and supports the expansion of vaccination, particularly among younger age groups.”
Meanwhile, the low rate of effectiveness among seniors “reinforces the need for continued advances in influenza vaccines, especially to increase protective benefits for older adults.”
“This finding should not discourage future vaccination by [people ages 65 and older], who are at greater risk for more severe cases and complications from influenza,” the authors wrote. “Influenza vaccines remain the best preventive tool available, and [vaccine effectiveness]is known to vary by virus type/subtype, age group, season, host immunity and the outcome measured.”
The authors noted the vaccine effectiveness estimates in the report are limited to the prevention of outpatient medical visits instead of more severe illness outcomes such as hospitalization or death.
To clinicians, the authors advise maintaining “a high index of suspicion for influenza infection among persons with acute respiratory illness while influenza activity is ongoing. Early antiviral treatment can reduce influenza-associated illness severity and complications.”
The authors recommend “initiating antiviral medications for patients with suspected influenza, regardless of their influenza vaccination status,” if they are 65 or older, hospitalized, have progressive or complicated illness or otherwise are at higher risk for complications from influenza.
“Antiviral treatment can be initiated empirically, preferably within 48 hours after illness onset, and should not be delayed pending confirmatory diagnostic testing nor be dependent upon tests with limited sensitivity (e.g. negative rapid tests). Among hospitalized patients, treatment should be initiated on admission.”
The vaccine effectiveness estimates are not final, according to the report. An increased sample size and adjustment for additional potential confounders (such as chronic medical conditions and functional status) at the end of the season could change them.
The full report is available in the Feb. 22 edition of the Morbidity and Mortality Weekly Report at www.cdc.gov/mmwr/preview/mmwrhtml/mm6207a2.htm?s_cid=mm6207a2_w.