Assessing the role of contact tracing in a suspected H7N2 influenza A outbreak in humans in Wales

Abstract

Background: The detailed analysis of an outbreak database has been undertaken to examine the role of contact tracing in controlling an outbreak of possible avian influenza in humans. The outbreak, initiating from the purchase of infected domestic poultry, occurred in North Wales during May and June 2007. During this outbreak, extensive contact tracing was carried out. Following contact tracing, cases and contacts believed to be at risk of infection were given treatment/prophylaxis.

Methods: We analyse the database of cases and their contacts identified for the purposes of contact tracing in relation to both the contact tracing burden and effectiveness. We investigate the distribution of numbers of contacts identified, and use network structure to explore the speed with which treatment/prophylaxis was made available and to estimate the risk of transmission in different settings.

Results: Fourteen cases of suspected H7N2 influenza A in humans were associated with a confirmed outbreak among poultry in May-June 2007. The contact tracing dataset consisted of 254 individuals (cases and contacts, of both poultry and humans) who were linked through a network of social contacts. Of these, 102 individuals were given treatment or prophylaxis. Considerable differences between individuals' contact patterns were observed. Home and workplace encounters were more likely to result in transmission than encounters in other settings. After an initial delay, while the outbreak proceeded undetected, contact tracing rapidly caught up with the cases and was effective in reducing the time between onset of symptoms and treatment/prophylaxis.

Conclusions: Contact tracing was used to link together the individuals involved in this outbreak in a social network, allowing the identification of the most likely paths of transmission and the risks of different types of interactions to be assessed. The outbreak highlights the substantial time and cost involved in contact tracing, even for an outbreak affecting few individuals. However, when sufficient resources are available, contact tracing enables cases to be identified before they result in further transmission and thus possibly assists in preventing an outbreak of a novel virus.

Figure 1

Outbreak network. Network showing the individuals and poultry involved in the outbreak and the links identified between them. The network contains 254 individuals, (including 2 cases in poultry (squares), 14 human cases, and 72 other individuals from whom contact tracing was carried out). Cases and links between cases are shown in red. Those other individuals from whom contact tracing was carried out and their links to cases are shown in blue. Other individuals and links are shown in grey.

Figure 2

Population variability in number of contacts identified. For each point on the x-axis, the black line shows the number of people (out of the 86 individuals from whom contact tracing was carried out) who identified at least that many contacts. The red, green, and blue lines show the same for home, work, and other contacts respectively.

Figure 3

Actual and idealised time courses of treatment and tracing. Time course of tracing and treatment/prophylaxis during the outbreak (blue). Also shown are two idealised situations: one - instantaneous tracing (green) - in which tracing and treatment/prophylaxis take place immediately a contact is named; the other (red) in which there is both instantaneous tracing and perfect surveillance, such that as soon as individuals become symptomatic they are treated and their contacts are identified and traced.

Figure 4

Time-ordered network of linked cases showing estimated secondary infections in absence of treatment. The 14 human cases (circles), along with the infected market and flock (squares), plotted from left to right in time ordered position; also shown are traced links between them categorised as home (red), work (green) and other (blue). Links involving birds are in black. The colour of a circle shows the length of time between the onset of symptoms and treatment being given, while the area is proportional to the approximate expected number of subsequent cases had they not been treated.