Don't Discount Low Efficiency Influenza Vaccines

A new study finds that even in a low-efficacy year, the flu vaccine can have important public health benefits, so long as its deployment is strategic and widespread.

A cross-country team of researchers used an optimization algorithm to analyze a mathematical model of influenza patterns across the United States. Their goal was to find out the optimal age-specific distribution of the vaccine. With optimal distribution, even low-efficacy flu vaccines can make a difference, although their benefits vary somewhat depending on which age group has a high uptake rate.

Corresponding study author Burton H. Singer, PhD, of the Emerging Pathogens Institute of the University of Florida, said this study aligns with other research hinting that the flu vaccine is impactful even if a particular year’s vaccine does not have a high success rate against the virus.

“I first became interested in the potential of low-efficacy vaccines nearly 10 years ago,” he told Contagion^®. “Recently there was some empirical evidence that there could be benefits. This certainly provoked our taking a close look at the problem.”

Dr. Singer and his colleagues have now provided what is believed to be the first population-wide look at the impact of low-efficacy vaccines with a focus on population-based optimization. They looked not only at incidences of influenza infection, but also at hospitalization, mortality, and disease-adjusted-life-years.

When a given year’s vaccine’s efficacy is low, the investigators found that the best way to reduce incidences and hospitalizations due to the flu is to vaccinate school-aged children and adults in their 30s. Because those groups have disproportionately high transmission rates, vaccinating those populations helps protect a broad band of the population.

To reduce mortality, the best approach is to boost the rates of vaccination among the elderly, offering direct protection against the virus to as many members of that vulnerable population as possible.

Notably, the investigators found that mortality and overall health burden were more sensitive to vaccine coverage levels than to the efficacy of a given year’s vaccine.

Dr. Singer said he was surprised by the extent to which coverage had such an effect.

“You really had to do the modeling to see it clearly,” he said. “How general this phenomenon is across different diseases is, to the best of my knowledge, unknown.”

The team hopes this modeling helps to make the case that the United States needs to do a much better job of deploying the flu vaccine broadly across population groups. That isn’t always easy when the impact of any given individual getting a flu shot might primarily be felt on the population level, rather than on the individual level. However, Dr. Singer said physicians need to speak with patients to make sure they understand herd immunity.

“The doctor would have to make the case that if a large majority get vaccinated, then everybody gains,” he said. “The potential for less severe clinical outcomes for the individual is also there. Herd immunity isn't enough to protect the elderly, where mortality is the main threat. However, as we show, vaccination of this group in the presence of low efficacy vaccines can increase survival chances.”