Clinical and epidemiological features of 2009 pandemic H1N1 influenza differ slightly according to seroprevalence status during the second wave in the general population in México.

BACKGROUND: Clinical features of pandemic H1N1 have been derived from lab-confirmed, hospitalized, or critically ill subjects. This report describes the clinical features of H1N1 and their prevalence from non-confirmed subjects according to seroprevalence status in México. The objective was to determine the prevalence of these clinical features from non-confirmed cases of pandemic H1N1 and to compare them according to seroprevalence status in northern Monterrey, México, during 2009, and to identify the predictive signs and symptoms; there have been no prior serologic studies in México. METHODS: During November-December 2009, 2,222 volunteers, ages 6-99 years, were categorized into 3 symptomatic groups: influenza-like illness, respiratory illness, and non-respiratory illness. Antibodies against influenza A/H1N1/2009 were determined by a virus-free enzyme-linked immunosorbent assay (ELISA) method. Demographics and clinical presentation were assessed through face-to-face questionnaire, and the association with seroprevalence status was determined and compared. RESULTS: Overall seroprevalence was 39%. Of the seropositive subjects, 67% were symptomatic and 33% were asymptomatic. Seventy-one percent of seropositive symptomatic subjects reported respiratory illness, 17% reported non-respiratory symptoms, and 12% reported influenza-like illness. The most common symptoms were rhinorrhea/nasal congestion (93%) and headache (83%). No significant difference was found between the symptom profiles of the seropositive group, compared to the seronegative one, nor of the median duration of symptoms. The seropositive group had a significantly elevated proportion of influenza-like illness (12%), compared to the seronegative group (8%). The proportion of subjects who took days off and who sought medical attention was significantly higher in the seropositive group. No single symptom was associated as a predictor of seropositiveness. CONCLUSIONS: One third of the seropositive subjects were asymptomatic, and few had an influenza-like illness. No difference was found in the symptom profiles of the seropositive and seronegative groups. No single symptom predicted seropositiveness. Large scale population studies are needed, especially in México, to characterize clinical syndromes.

Seroprevalence of antibodies to influenza A/H1N1/2009 among transmission risk groups after the second wave in Mexico, by a virus-free ELISA method.

OBJECTIVE: No serological studies have been performed in Mexico to assess the seroprevalence of influenza A/H1N1/2009 in groups of people according to the potential risk of transmission. The aim of this study was to determine the seroprevalence of antibodies against influenza A/H1N1/2009 in subjects in Mexico grouped by risk of transmission. METHODS: Two thousand two hundred and twenty-two subjects were categorized into one of five occupation groups according to the potential risk of transmission: (1) students, (2) teachers, (3) healthcare workers, (4) institutional home residents aged >60 years, and (5) general population. Seroprevalence by potential transmission group and by age grouped into decades was determined by a virus-free ELISA method based on the recombinant receptor-binding domain of the hemagglutinin of influenza A/H1N1/2009 virus as antigen (85% sensitivity; 95% specificity). The Wilson score, Chi-square test, and logistic regression models were used for the statistical analyses. RESULTS: Seroprevalence for students was 47.3%, for teachers was 33.9%, for older adults was 36.5%, and for the general population was 33.0%, however it was only 24.6% for healthcare workers (p=0.011). Of the students, 56.6% of those at middle school, 56.4% of those at high school, 52.7% of those at elementary school, and 31.1% of college students showed positive antibodies (p<0.001). Seroprevalence was 44.6% for college teachers, 31.6% for middle school teachers, and 29.8% for elementary school teachers, but was only 20.3% for high school teachers (p=0.002). CONCLUSIONS: The student group was the group most affected by influenza A/H1N1/2009, while the healthcare worker group showed the lowest prevalence. Students represent a key target for preventive measures.