During recent U.S. flu seasons, children and adults who contracted influenza despite vaccination had significantly fewer severe infections and infection complications, compared with unimmunized people, according to two separate reports from CDC researchers presented at an annual scientific meeting on infectious diseases.
One of the reports tracked the impact of flu vaccine in children using data that the CDC collected at seven medical centers that participated in the agency’s New Vaccine Surveillance, which provided information on children aged 6 months to 17 years who were hospitalized for an acute respiratory illness, including more than 1,700 children during the 2016-2017 flu season and more than 1,900 during the 2017-2018 season. Roughly 10% of these children tested positive for influenza, and the subsequent analysis focused on these cases and compared incidence rates among children who had been vaccinated during the index season and those who had remained unvaccinated.
Combined data from both seasons showed that vaccinated children were 50% less likely to have been hospitalized for an acute influenza infection, compared with unvaccinated kids, a pattern consistently seen both in children aged 6 months to 8 years and in those aged 9-17 years. The pattern of vaccine effectiveness also held regardless of which flu strain caused the infections, reported Angela P. Campbell, MD, a CDC medical officer.
“We saw a nice benefit from vaccination, both in previously healthy children and in those with an underlying medical condition,” a finding that adds to existing evidence of vaccine effectiveness, Dr. Campbell said in a video interview. The results confirmed that flu vaccination does not just prevent infections but also cuts the rate of more severe infections that lead to hospitalization, she explained.
Another CDC study looked at data collected by the agency’s Influenza Hospitalization Surveillancefrom adults at least 18 years old who were hospitalized for a laboratory-confirmed influenza infection during five flu seasons, 2013-2014 through 2017-18. The data, which came from more than 250 acute-care hospitals in 13 states, included more than 43,000 people hospitalized for an identified influenza strain and with a known vaccination history who were not institutionalized and had not received any antiviral treatment.
After propensity-weighted adjustment to create better parity between the vaccinated and unvaccinated patients, the results showed that people 18-64 years old with vaccination had statistically significant decreases in mortality of a relative 36%, need for mechanical ventilation of 34%, pneumonia of 20%, and need for ICU admission of a relative 19%, as well as an 18% drop in average ICU length of stay, Shikha Garg, MD, said at the meeting. The propensity-weighted analysis of data from people at least 65 years old showed statistically significant relative reductions linked with vaccination: 46% reduction in the need for mechanical ventilation, 28% reduction in ICU admissions, and 9% reduction in hospitalized length of stay.
Further analysis of these outcomes by the strains that caused these influenza infections showed that the statistically significant benefits from vaccination were seen only in patients infected with an H1N1 strain. Statistically significant effects on these severe outcomes were not apparent among people infected with the H3N2 or B strains, said Dr. Garg, a medical epidemiologist at the CDC.
“All adults should receive an annual flu vaccination as it can improve outcomes among those who develop influenza despite vaccination,” she concluded.
Results from a third CDC study reported at the meeting examined the importance of two vaccine doses (administered at least 4 weeks apart) given to children aged 6 months to 8 years for the first season they receive flu vaccination, which is the immunization approach for fluby the CDC. The findings from a total of more than 7,500 children immunized during the 2014-2018 seasons showed a clear increment in vaccine protection among kids who received two doses during their first season vaccinated, especially in children who were 2 years old or younger. In that age group, administration of two doses produced vaccine effectiveness of 53% versus a 23% vaccine effectiveness after a single vaccine dose, reported Jessie Chung, a CDC epidemiologist.