BACKGROUND:
Influenza vaccination is the most effective intervention to prevent influenza virus infections. Vaccine effectiveness (VE) can vary due to factors such as matching between vaccine strains and prevailing strains, age and other characteristics of the vaccine recipients.
OBJECTIVE:
To evaluate influenza VE against medically-attended illness in different age groups and against specific influenza types/subtypes in Hong Kong.
METHODS:
A test-negative study was conducted from December 2014 through August 2017 in 20 outpatient clinics. Patients at least 6?months of age presenting with at least two symptoms of acute respiratory illness, ARI (fever ≥37.8?°C, cough, sore throat, runny nose, headache, myalgia and phlegm) within 72?h of onset were tested for influenza virus by reverse transcription polymerase chain reaction (PCR). Vaccination history was assessed by self-report or medical records at the clinics. VE against medically-attended illness was estimated using conditional logistic regression for influenza PCR result versus vaccination history, matching by calendar time and adjusting for age, age-squared, sex, and chronic medical illness. Additional analyses examined VE by age group and by influenza type/subtype.
RESULTS:
We enrolled 2566 patients, of whom 1118 (43.6%) tested positive for influenza A or B virus by PCR. Test-positive subjects were generally older, more likely to present with one of the symptoms of ARI, and less likely to receive vaccination against influenza. VE estimates for influenza A(H1N1), A(H3N2), B/Yamagata and B/Victoria were 61.6% (95% confidence interval, CI: 21.8%, 81.1%), 26.4% (95% CI: -1.3%, 46.6%), 67.0% (95% CI: 25.9%, 85.3%), 60.4% (95% CI: 0.3%, 84.3%), respectively. Estimates of VE by age group were generally higher in adults aged 50-64 and lower among children and older adults.
CONCLUSIONS:
VE against medically-attended influenza was moderate in Hong Kong, confirming the impact of influenza vaccination in reducing disease burden. The reduced VE for influenza A(H3N2) is a continuing concern.