From 26 April (week 17) up to 3 October 2009 (week 40), samples of 875 patients were tested in the lab. According to the Influenza Preparedness Plan, only patients fulfilling criteria for a possible case should have been selected for laboratory testing. The fact that only 38% of patients tested complied with the definition of a possible case, indicates that the recruitment of cases for lab testing was not specific.
From 26 April to 13 July 2009 (containment phase), 123 (20%) out of 614 collected samples from suspected Influenza A(H1N1)2009 cases were confirmed by RT-PCR while from 14 July up to end September 2009 (mitigation phase) a similar proportion of 50 patients (19%) were diagnosed as Influenza A(H1N1)2009 cases among 261 tests performed (Figure ).
Number of lab tests by week and by result, week 17-40, 2009, Belgium. Other influenza includes the unsubtyped influenza A samples.
Seasonal flu also affected 16 cases, including 15 Influenza A(H3N2) virus and 1 Influenza B virus (1.8% of tested patients) and among them, 15 were identified during the containment phase.
In Belgium, the first positive case of Influenza A(H1N1)2009 was identified from a sample taken on 12 May during week 20. After 8 weeks a total of 123 positive cases for Influenza A(H1N1)2009 were laboratory confirmed in Belgium. The official number of cases for this period reached 126 patients since one case was registered by the SGP network and two cases remained probable cases. These two symptomatic cases were close contacts of confirmed cases but no samples were taken for laboratory confirmation because the patients were young children.
A peak was observed in week 28 when cases related to two outbreaks were detected with 13 confirmed cases among participants of a rock festival in Werchter [5
] and 15 confirmed cases from a language summer school. Afterwards a constant and low viral activity was observed during the summer holiday period.
The first death was reported in week 30 and the second one in week 38, both during the mitigation period. For both phases, 176 cases were diagnosed as Influenza A(H1N1)2009 and among them males were more affected than females (sex ratio (M/F)= 1.5). The median age of the Influenza A(H1N1)2009 cases was 21 years (range 1-59), significantly lower (p<0.01) than the median age among tested patients (32 years) and still lower than the median age for seasonal influenza affected patients (41 years).
The detection rate of Influenza A(H1N1)2009 in Belgium between week 17 and 40 was about 1.6 cases by 100,000 inhabitants. The most affected age group by Influenza A(H1N1)2009 was 15-24 years (Figure ) with 4.7 confirmed cases per 100,000. No reported cases were older than 59 years.
Number of lab tests per 100,000, by age group and by result, week 17-40, 2009, Belgium. Other influenza includes the unsubtyped influenza A samples.
The majority of cases were residents of the provinces Antwerp (Antwerpen), Flemish Brabant (Vlaams-Brabant) and the Brussels-Capital Region (Table ). Except in the Brussels Region, a decrease of the number of tests and of the incidence of Influenza A(H1N1)2009 was observed between the two phases. During the second phase the highest incidence remained in the province of Antwerp and the Brussels Region; both with densely and internationally populated cities.
Number of probable and confirmed cases and other results of lab tests, proportion by 100,000 inhabitants, by province, week 17-40/2009, Belgium
During the containment phase clinical criteria included fever > 38°C, myalgia or general discomfort and cough together with an epidemiological link. Once the circulation of the pandemic virus has been demonstrated in the Belgium population, the epidemiological link was not required anymore to classify a suspected patient as a possible case. As the lab questionnaire was not modified, this information was still mentioned in 77% of the lab questionnaires.
About 78% of the patients had fever, cough and myalgia. Dyspnoea was mentioned by 26% of the confirmed cases. About 30% of the confirmed cases mentioned an associated symptom and the most frequent ones were diarrhoea, possibly associated with nausea (11%), and sore throat (7%).
The classification of all patients following the criteria of the possible case definition allows calculating the sensitivity (76.4%; 95% CI 68.5-82.9) and specificity (64.5%; 95% CI 60.5-68.4) of the case definition. If the consequent positive predictive value is quite low, this result offered a negative predictive value of 91.8% (95% CI 88.6-94.2).
In both phases the most frequent epidemiological link was related to travel in an endemic area. The proportion of positive Influenza A(H1N1)2009 was lower among the 'travel related' compared with people coming into 'contact with case' or involved in an 'outbreak' (Table ) and the proportion is unchanged between both phases.
Repartition of the number of tests and results by type of risk, week 17-40/2009, Belgium
The country associated with the risk differed nevertheless according to the evolution of the pandemic (Figure ). During the containment phase, suspected and possible cases returned mainly from North America (35%) while this place of travel was related in only 2% of the suspected or possible cases during the mitigation phase. During the latter, suspected cases were mainly associated with travel within Europe or had not travelled. Two peaks in the number of tests in people who did not travel were associated in time with the two outbreaks.
Epidemiological links of tested patients, week 17-40/2009, Belgium.
Suspected patients included in the screening returned from more than 50 different countries of all continents. Most of them returned from North America (24.6%), followed by Europe (22.4%). About 50 tests were performed for patients back from Mexico none of them was positive for Influenza A(H1N1)2009.