Validation of diagnostic tests for detection of avian influenza in vaccinated chickens using Bayesian analysis

Impacts of a mass vaccination campaign against pandemic H1N1 2009 influenza in Taiwan: a time-series regression analysis

OBJECTIVES

A multicenter, hospital-wide, clinical and epidemiological study was conducted to assess the effectiveness of the mass influenza vaccination program during the 2009 H1N1 influenza pandemic, and the impact of the prioritization strategy among people at different levels of risk.

METHODS AND RESULTS

Among the 34 359 medically attended patients who displayed an influenza-like illness and had a rapid influenza diagnostic test (RIDT) at one of the three participating hospitals, 21.0% tested positive for influenza A. The highest daily number of RIDT-positive cases in each hospital ranged from 33 to 56. A well-fitted multiple linear regression time-series model (R(2)=0.89) showed that the establishment of special community flu clinics averted an average of nine cases daily (p=0.005), and an increment of 10% in daily mean level of population immunity against pH1N1 through vaccination prevented five cases daily (p<0.001). Moreover, the regression model predicted five-fold or more RIDT-positive cases if the mass influenza vaccination program had not been implemented, and 39.1% more RIDT-positive cases if older adults had been prioritized for vaccination above school-aged children.

CONCLUSIONS

Mass influenza vaccination was an effective control measure, and school-aged children should be assigned a higher priority for vaccination than older adults during an influenza pandemic.

Serological diagnosis of avian influenza in poultry: is the haemagglutination inhibition test really the 'gold standard'?

Background  The serological diagnosis of avian influenza (AI) can be performed using different methods, yet the haemagglutination inhibition (HI) test is considered the ‘gold standard’ for AI antibody subtyping. Although alternative diagnostic assays have been developed, in most cases, their accuracy has been evaluated in comparison with HI test results, whose performance for poultry has not been properly evaluated.

Objective  The objective of this study was to estimate the diagnostic sensitivity (Se) and specificity (Sp) of the HI test and six other diagnostic assays for the detection of AI antibodies without assuming a gold standard.

Methods  We applied a Bayesian version of latent class analysis to compare the results of multiple tests from different study settings reported in the literature.

Results  The results showed that the HI test has nearly perfect accuracy (i.e. 98·8% sensitivity and 99·5% specificity). It performed well in both chickens and turkeys and yet was less accurate in experimentally infected poultry, compared to naturally infected. Blocking ELISA and the indirect immunofluorescence assay also performed very well.

Conclusions  Given its very high Se and Sp, the HI test may be effectively considered a gold standard. In the framework of LPAI surveillance, where large numbers of samples have to be processed, the blocking ELISA could be a valid alternative to the HI test, in that it is almost as sensitive and specific as the HI test yet quicker and easier to automate.

Evaluating surveillance strategies for the early detection of low pathogenicity avian influenza infections

In recent years, the early detection of low pathogenicity avian influenza (LPAI) viruses in poultry has become increasingly important, given their potential to mutate into highly pathogenic viruses. However, evaluations of LPAI surveillance have mainly focused on prevalence and not on the ability to act as an early warning system. We used a simulation model based on data from Italian LPAI epidemics in turkeys to evaluate different surveillance strategies in terms of their performance as early warning systems. The strategies differed in terms of sample size, sampling frequency, diagnostic tests, and whether or not active surveillance (i.e., routine laboratory testing of farms) was performed, and were also tested under different epidemiological scenarios. We compared surveillance strategies by simulating within-farm outbreaks. The output measures were the proportion of infected farms that are detected and the farm reproduction number (R_h). The first one provides an indication of the sensitivity of the surveillance system to detect within-farm infections, whereas R_h reflects the effectiveness of outbreak detection (i.e., if detection occurs soon enough to bring an epidemic under control). Increasing the sampling frequency was the most effective means of improving the timeliness of detection (i.e., it occurs earlier), whereas increasing the sample size increased the likelihood of detection. Surveillance was only effective in preventing an epidemic if actions were taken within two days of sampling. The strategies were not affected by the quality of the diagnostic test, although performing both serological and virological assays increased the sensitivity of active surveillance. Early detection of LPAI outbreaks in turkeys can be achieved by increasing the sampling frequency for active surveillance, though very frequent sampling may not be sustainable in the long term. We suggest that, when no LPAI virus is circulating yet and there is a low risk of virus introduction, a less frequent sampling approach might be admitted, provided that the surveillance is intensified as soon as the first outbreak is detected.

Variability of NS1 proteins among H9N2 avian influenza viruses isolated in Israel during 2000-2009

The main aims of the present study were to characterize NS1 protein from H9N2 avian influenza viruses (AIVs) isolated in Israel and to investigate the possibility to use NS1-based indirect ELISA. To achieve these purposes, the non-structural gene (NS1) of 79 AIVs of the H9N2 subtype isolated in Israel in 2000-2009 was sequenced and genetically analyzed. The phylogenetic analysis demonstrated that four distinct introductions of H9N2 occurred in Israel during this period. Analysis of the inferred amino acid sequences of the NS1 proteins showed high, about 10%, differences between viruses of the 3rd and 4th introductions. Antibodies against NS1 protein in immune sera were tested by means of indirect ELISA using recombinant NS1 as antigen. Immune sera were obtained from experimentally H9N2-infected chicken after infection on 4, 7, 10, 14, and 21 days. All sera from chickens experimentally infected with 3rd- or 4th-introduction AIV contained anti-NS1 antibodies that were detected by enzyme-linked immunosorbent assay (NS1-ELISA) even though the recombinant NS1 used as antigen for NS1-ELISA differed significantly in its amino acid sequences from the NS1 protein of AIV that caused infection in experimental birds. These findings indicate that the sites of the NS1 protein by which viruses belonging to 3rd and 4th introduction are out of antigenic epitope positions were responsible for the results of NS1-based iELISA.