The transmission of A(H1N1)pdm09-person to person-is similar to that of other seasonal influenza viruses. Influenza viruses spread rapidly through infected air-borne droplets that are generated by coughing or sneezing. Hands are a major alternative route of viral spread, and can be contaminated “directly”, when droplets land on the hands from an infected person coughing nearby, or “indirectly”, when hands pick up the virus by touching contaminated objects or surfaces. The infectious period for a confirmed case is from one day before the onset of symptoms to seven days after onset [1]–[2].

As concerns community-wide diffusion, influenza epidemics are widespread outbreaks of highly contagious respiratory disease that appear suddenly. Influenza pandemics are characterized by the rapid worldwide spread of a virus to which humans have had no previous exposure.

In infectious disease epidemiology, diffusion is an important concept depicting the dynamics of the spread of a microorganism through time and space [3]. To understand the spread of influenza in a community, it is necessary to study epidemiological and virological conditions and the geographical determinants of influenza during a pandemic [4]. Diverse factors influence the spread of epidemics through human populations, particularly the characteristics of the pathogen responsible for the infection [5], human mobility patterns [6], [7], the sociodemographic structure of the population [4], [8] and intervention measures.

On a population level, influenza spreads both by contagious diffusion (wave-like from one or more central foci) and hierarchical diffusion (movement from larger to smaller towns) [9]. The spread of influenza epidemics is linked more to the rates of movement of people to and from work than to geographical distance or air travel [10]. Also, the virus spreads more rapidly in more densely populated locations.

Recently, the A(H1N1)pdm09 influenza virus swept rapidly across the world after its first detection in humans in April 2009 in Mexico and the US [11], [12]. The A(H1N1)pdm09 pandemic virus is now well-characterized biologically, clinically and epidemiologically [13]. However, little is known about the timing and impact of pandemic influenza in Africa.

The first reported case of A(H1N1)pdm09 influenza to be imported into Madagascar was August 12^th, 2009. The first laboratory-confirmed cases without travel history were detected on October 8^th, 2009: three teenagers attending one of the largest schools in Antananarivo [14]. The first wave of A(H1N1)pdm09 peaked in November 2009 in Antananarivo.

Here, we report an analysis of the diffusion pattern of influenza A(H1N1)pdm09 virus by examining all clinical cases reported to the sentinel network. This network allowed the pandemic to be monitored nationwide in real time.

We studied the epidemiology of the current pandemic of influenza A(H1N1)pdm09 virus in Madagascar, including the temporal and geographic pattern of spread and the clinical characteristics of A(H1N1)pdm09 disease among ILI patients. We characterized the range and chronology of the pandemic in Madagascar with real-time data from a sentinel network.

This is the first description of overall trends of pandemic influenza A(H1N1)pdm09 infection in Madagascar. This country was one of the first countries in Africa to report laboratory-confirmed cases of this new influenza virus: cases of pandemic influenza were detected in Madagascar before mass gatherings occurred. All of the initial sporadic cases identified were imported (from India, Mauritius and Reunion Island). The sentinel surveillance networks which have been operational since 2007 [15] presumably contributed to this because coverage was good in high density population settings; the updated information given to health care professionals in June 2009 during an influenza workshop organized by Institut Pasteur from Madagascar may also have contributed.

The pattern of spread of A(H1N1)pdm09 in Madagascar was dominated by a wave that emanated from the capital, Antananarivo. The early dynamics of this wave might have been associated with a high frequency of international travel, increasing the risks of a major epidemic in the capital city [10].

It was challenging to implement the early phase of the WHO recommendations concerning the epidemic. Thus, epidemiological investigation was crucial to monitor the epidemic better, particularly to identify risk groups and factors that contributed to the development of the epidemic. An understanding of the epidemiology of past pandemics, and in particular the last A(H1N1)pdm09 pandemic, may help health authorities to prepare and implement response programs to subsequent waves and pandemics [16].

Collecting relevant information on pre-existing chronic conditions and complications among hospitalized cases could be valuable to fill the gaps in the existing epidemiological data. However, in Madagascar, like in others developing countries, the level of hospital care is poor, and there is a lack of influenza surveillance in hospitals (e.g. concerning severe cases). This weakness with regard to surveillance, and hospitalization costs for patients, need to be considered and overcome so as strengthen surveillance systems with data from hospitals.

Our report demonstrates that the signs and symptoms in ILI or confirmed cases are similar to those observed in patients with seasonal influenza [16], [17]. We observed that sampling children aged under 1 year old was much less effective than that for other groups. The proportions of patients sampled did not differ between the other age groups, and the positivity rate for A(H1N1)pdm09 was high in patients aged between 4 and 15 years. One characteristic of this pandemic is that it disproportionately affected children and young adults [18]; indeed, a study in the USA reported that 60% of confirmed A(H1N1)pdm09 cases were aged 18 years or younger [19].

We also observed that transmission of the influenza A(H1N1)pdm09 virus during the pandemic in Madagascar, like in other countries, appeared to be inevitable, presumably due to the nature of the infection: the virus spread despite preparations to mitigate the situation, including systematic detection of all suspected cases, social distancing options (such as school closure) and antiviral treatment of all confirmed cases and their contacts during the early stage of the epidemic. However, in Madagascar like in other remote areas, the impact of vaccination or timely oseltamivir use on influenza transmission was probably low because these measures were unavailable throughout the country.

As in other countries [20]–[22], there were regional differences in the trend of the epidemic and the timeline of A(H1N1)pdm09 spread, and the causes are not easily explained. These differences may be associated with the demographic structure or population density of these regions. The region specificity of the pattern requires further examination.

Data from Madagascar, like those from others countries [23], suggest that the A(H1N1)pdm09 virus spreads rapidly through communities once introduced from an affected area. For the specific case of Madagascar Island, the principal spread of infection took approximately 3–4 months. This rapid spread of pandemic influenza infection across the whole country underscores the need for real-time surveillance systems to track viral activity across regions and districts.

In conclusion, the sentinel data allowed description of the pattern of disease activity. Both seasonal and pandemic influenza surveillance, using sentinel data, is informative when combined with laboratory testing. We advocate (i) enhancing the surveillance capacity with the aim of mitigating the course of future epidemics early, (ii) strengthening surveillance efforts, and (iii) promoting information sharing in Africa because the influenza epidemiology on the continent is largely unknown. In addition population-based serological surveys should be performed to generate more accurate estimates of the epidemic's impact.