Influenza Vaccine Effectiveness in the United States — 2015/16 Season

Published - Written by Karen Sokal-Gutierrez, MD, MPH

A 5-year-old boy is seen in the primary care pediatric office in October. His mother says, “Since starting kindergarten, Sam has had one cold after another, and then the whole family gets sick.” She adds, “He just got all of his shots for kindergarten and received the nasal spray against flu when he was in preschool. What do you recommend for the flu vaccine this year?”

Although colds and flu are usually mentioned in the same breath, it’s important to remind patients that influenza — which circulates every year, typically between October and May in the Northern Hemisphere — can cause severe illness and even death, especially in people with cardiorespiratory or immunosuppressed conditions. The good news is that there’s an annual flu vaccine to help prevent it.

The flu vaccine was initially promoted for the elderly, but recognition of the morbidity and mortality associated with influenza in children — and transmission in families, child care, and schools — has led to the promotion of flu vaccine for children. The American Academy of Pediatrics (AAP) and Centers for Disease Control and Prevention (CDC) now recommend annual seasonal influenza immunization for all people aged 6 months and older. Both trivalent and quadrivalent vaccines have been developed and two vaccine preparations are available for children: Live attenuated influenza vaccine (LAIV) administered by nasal spray and inactivated influenza vaccine (IIV) administered by intramuscular injection. Since children and parents prefer to avoid shots, the intranasal flu vaccine had been recommended for children who were not at high risk for influenza infection and complications. However, after initial reports of the success of the intranasal LAIV, the Influenza Vaccine Effectiveness Network found that the 2013–2014 LAIV lacked effectiveness in young children. As a result, the influenza virus strain was changed for the 2015–2016 season.

This week, NEJM published a study led by researchers from the CDC on the effectiveness of the influenza vaccine in the U.S. during the 2015–2016 season in people aged >6 months. All study participants had sought medical care for acute respiratory illness and cough lasting 7 days or more in November through  mid-April at clinics in Michigan, Pennsylvania, Texas, Washington, and Wisconsin. Nasal and oropharyngeal swab specimens were used to test for influenza A and B virus using real-time reverse transcriptase polymerase chain reactions (RT-PCR). Influenza vaccine effectiveness was estimated using a test-negative design, comparing the odds of testing positive for influenza in  vaccinated versus unvaccinated subjects.

Among the 6879 participants, 1309 (19%) tested positive for influenza, predominantly A(H1N1)pdm09 (11%) and influenza B (7%). Overall vaccine effectiveness for preventing any influenza illness was 48% (95% CI, 41%–55%). Among children aged 2–17 years, effectiveness for the IIV vaccine was 60% (95% CI, 47-70%), while the intranasal LAIC was not effective (5%; 95% CI, -47%–39%).

The authors concluded that influenza vaccines reduced the risk of influenza illness in 2015–2016. However, while the flu shot was effective, the intranasal vaccine was again ineffective in children. The authors hypothesized that “poor replicative fitness of the LAIV4 A(H1N1)pdm09 strains or vaccine-virus interference” reduced the effectiveness of LAIV. Since the same LAIV  strain was planned for 2016–2017, the Advisory Committee on Immunization Practices recommended against the use of LAIV for 2016–2017. In September 2016, the headline of an AAP news article on the intranasal vaccine (marketed as FluMist) stated: "Intranasal FluMISSED its target."

Dr. Lindsey Baden, an NEJM Deputy Editor and infectious disease specialist commented, “Developing a highly effective influenza vaccine remains a significant challenge. We must also develop delivery systems that are more tolerable but do not impact vaccine efficacy and effectiveness, especially in our most vulnerable patients.”

Returning to the question posed by Sam’s mother: The primary care provider should recommend the annual flu vaccine, explaining that the flu shot can cut the risk of flu by one-half, while the nasal vaccine does not appear to be effective in children. Additional recommendations include flu shots for all family members and teaching everyone to cover their coughs and wash hands frequently with soap and water.

 

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Dr. Sokal-Gutierrez is Physician at UCSF - UC Berkeley Joint Medical Program.