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Seasonal flu vaccine does not significantly impact efficacy of pandemic vaccine

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October 9, 2018

The seasonal influenza vaccine had no significant clinical impact on the effectiveness of the 2009 pandemic influenza vaccine in patients who received both, according to findings from a recently published study.

Writing in Clinical Infectious Diseases, Rachel U. Lee, MD, division head for allergy and immunology in the department of internal medicine at Naval Medical Center San Diego, and colleagues said conflicting study results have raised questions about whether prior or co-receipt of the seasonal vaccine had an impact on the protectiveness of the vaccine that was developed especially for the 2009 influenza pandemic by boosting or diminishing its efficacy.

“In general, influenza continues to be a leading cause of illness and is a major public health concern. Although the most effective prevention strategy is vaccination, vaccine efficacy varies markedly from year to year depending on how well the vaccine matches the circulating strains, vaccine formulation, and other factors,” they wrote. “Thus, continued research into preventive strategies is needed to better understand the variables associated with effectiveness. A large segment of the population received the seasonal influenza vaccination; therefore, its role in the pandemic is clinically relevant.”

Lee and colleagues conducted an observational cohort study of United States military personnel aged 18 to 49 years who were on active duty and stationed in the contiguous U.S. during the pandemic. They reviewed electronic health and manpower databases — which include vaccination records — along with health care encounters for influenza-like illness (ILI), influenza and pneumonia, and examined rates by vaccination type to identify potential factors associated with the risk for study outcomes, primarily ILI events.

The results included 621,823 service members who received at least one influenza vaccine — 36,655 who received the seasonal vaccine only, 47,133 who received the pandemic vaccine only and 538,035 who received both.

Of the individuals who received at least one influenza vaccine, 43,757 were diagnosed with influenza-like illnesses. There were 853 cases of influenza and 1,259 cases of pneumonia among these individuals.

In the group receiving the seasonal vaccine only, there was an incidence rate of 0.79% for influenza and 0.52% for pneumonia. The group that received the pandemic vaccine alone had a rate of 0.09% for influenza and 0.19% for pneumonia. The group that received both had a rate of 0.10% and 0.18%.

The study showed that individuals receiving the pandemic vaccine, either alone or in addition to the seasonal vaccine, had an approximately 50% reduction in influenza-like illness, an 88% reduction in influenza and a 63% reduction in pneumonia compared with those who received only the seasonal vaccine. Lee and colleagues said there was no clinically significant difference in influenza-like illness, influenza or pneumonia attack rates among those receiving the pandemic vaccine with or without the presence of the seasonal vaccine. They added that there was also no clinically relevant difference in pandemic vaccine effectiveness between seasonal trivalent inactivated and live attenuated influenza vaccine recipients.

“During the 2009-2010 pandemic, the [pandemic vaccine] was effective in reducing rates of influenza-like illness, influenza and pneumonia in young healthy adults,” the authors concluded. “Administration of the seasonal vaccine should continue without concern of potential interference with a novel pandemic vaccine, though more studies are needed to determine if this is applicable to other influenza seasons.” – by Caitlyn Stulpin

Disclosures: The authors report no relevant financial disclosures.