Study finds 9-month drop-off in flu vaccine effectiveness in kids
In children, flu vaccine effectiveness declines over 9 months following immunization, according to a study in Hong Kong that took place over five flu seasons. A team based at the University of Hong Kong reported its findings on Nov 12 in The Lancet Respiratory Medicine.
Flu circulates almost year-round in Hong Kong, and the team used a test-negative case-control design to study vaccine effectiveness in children ages 6 months to 17 years, focusing on changes in the interval between vaccination and hospital admission.
Of 15,695 children hospitalized with respiratory infection from Sep 1, 2012, to Aug 31, 2017, 2,500 (15.9%) tested positive for flu and 13,195 (84.1%) tested negative. Of the vaccinated patients, 159 (6.4%) tested positive for flu and 1,445 (11%) tested negative.
Most of the children were vaccinated in December of each flu season. Pooled vaccine effectiveness declined over the following months, dropping from 79% for September through December, to 67% for January to April, to 43% for May to August. In a separate analysis, the investigators estimated that vaccine effectiveness dropped by 2 to 5 percentage points each month.
The team concluded that the findings support the importance of annual vaccination in children and the need for a flu vaccine that can provide broader, longer-lasting protection.
In a related commentary, two vaccine experts from Australia wrote that the findings have important policy implications and will likely raise questions about modeling studies that health planners use to predict the effect of vaccination strategies. They said uncertainties about the predicted effect of flu vaccines are especially relevant for tropical places like Hong Kong, where flu circulation is more unpredictable.
Nov 12 Lancet Respir Med abstract
Nov 12 Lancet Respir Med commentary
WHO: Global flu at interseasonal levels in much of the world
Most areas of the world reported interseasonal flu levels this past week with some regions reporting increased activity, according to the latest global flu update from the World Health Organization (WHO).
In most of the Northern Hemisphere and the temperate regions of the Southern Hemisphere, flu held at intersessional levels, with influenza A being the most widely circulating type. Several nations in Europe and North America saw a rise in reports of influenza-like illnesses.
Southeast Asia and India reported increased flu activity, as did several nations in northern and western Africa. Respiratory illness indicators started to increase in some west Asian countries, as well.
In the second half of October, the WHO said 86% of specimens positive for the flu were typed as influenza A and 14% as influenza B. Of the sub-typed influenza A viruses, 64.5% were 2009 H1N1 and 35.5% were H3N2. Of the characterized B viruses, 52.4% belonged to the B-Yamagata lineage and 47.6% to the B-Victoria lineage.
Nov 12 WHO update