Influenza B outbreak among influenza-vaccinated welfare home residents in Singapore.
BACKGROUND
Outbreaks of acute respiratory illness occur commonly in long-term care facilities (LTCF), due to the close proximity of residents. Most influenza outbreak reports have been from temperate countries. This study reports an outbreak of influenza B among a highly immunised resident population in a welfare home in tropical Singapore, and discusses vaccine efficacy and the role of acute respiratory illness surveillance for outbreak prevention and control.


METHODS
During the period from 16 to 21 March 2007, outbreak investigations and active case finding were carried out among residents and nursing staff at the welfare home. Interviews and medical notes review were conducted to obtain epidemiological and clinical data. Hospitalised patients were tested for respiratory pathogens. Further genetic studies were also carried out on positive respiratory samples.


RESULTS
The overall clinical attack rate was 9.4% (17/180) in residents and 6.7% (2/30) in staff. All infected residents and staff had received influenza immunisation. Fifteen residents were hospitalised, with 2 developing severe complications. Genetic sequencing revealed that the outbreak strain had an 8.2% amino acid difference from B/Malaysia/2506/2004, the 2006 southern hemisphere influenza vaccine strain, which the residents and staff had earlier received.


CONCLUSIONS
A mismatch between the vaccine and circulating influenza virus strains can result in an outbreak in a highly immunised LTCF resident population. Active surveillance for acute respiratory illness in LTCFs could be implemented for rapid detection of antigenic drift. Enhanced infection control and other preventive measures can then be deployed in a timely manner to mitigate the effect of any outbreaks.
Win
Mar Kyaw
MK
Clinical Epidemiology, Tan Tock Seng Hospital, Singapore.
Chow
Angela
A
Chen
Mark
M
Lau
Yuk Fai
YF
Ooi
Eng Eong
EE
Leo
Yee Sin
YS
D000328:Adult; D000368:Aged; D004196:Disease Outbreaks / Q000517:prevention & control*; D005260:Female; D006801:Humans; D009981:Influenza B virus / Q000276:immunology*; D007252:Influenza Vaccines / Q000627:therapeutic use; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control* / Q000821:virology; D007407:Interviews as Topic; D008297:Male; D008485:Medical Audit; D008875:Middle Aged; D009735:Nursing Homes; D012846:Singapore / Q000453:epidemiology; D012946:Social Welfare; D055815:Young Adult
D007252:Influenza Vaccines