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ISS
Istituto Superiore di Sanità
EpiCentro - L'epidemiologia per la sanità pubblica
Istituto Superiore di Sanità - EpiCentro


NATIONAL SURVEILLANCE - Influenza Surveillance in Italy during the 2000-2001 Season

Reported by Maria Cristina Rota for the FLU-ISS Group:

Epidemiologic surveillance: Director: Stefania Salmaso, Antonino Bella, Barbara de Mei, Stefania Giannitelli, Donatella Mandolini, Maria Cristina Rota; Virologic surveillance Director: Isabella Donatelli, Chiara Affinito, Laura Campitelli, Concetta Fagiani, Simone Fiaccamento, Simona Puzelli

 

To monitor the incidence of influenza-like illness and to examine the extent of the annual seasonal epidemic in Italy, the Istituto Superiore di Sanità (ISS) and the Centro Interuniversitario Genova-Milano di Ricerca sull’Influenza (Genoa-Milan Inter-University Center for Influenza Research; CIRI) organized, on a trial basis during the 1999-2000 season, a sentinel surveillance network based on general medicine physicians and pediatricians. Approximately 700 physicians from all 21 regions and autonomous areas voluntarily enrolled in the network and reported the number of cases of influenza seen in their practices on a weekly basis. Surveillance was conducted between the 42th week of 1999 and 17th week of 2000. In Italy, residents must register and seek routine care from one of the physicians employed or contracted by their Local Health Agency, making it possible to calculate overall incidence rates per 1000 enrolled patients as well as age-specific rates (0-14 years, 15-64 years, and ≥65 years) and to estimate the coverage of the surveillance system. During 1999-2000, the network contained an estimated 1.5% of the total population of Italy.

 

During the first year of surveillance, the sentinel network demonstrated itself to be an essential tool to describe temporal trends on influenza-like illness in Italy and to construct a baseline against which future years could be compared. For this reason, and because influenza continues to constitute an important public health problem, the surveillance system was institutionalized in 2000 with the collaboration of the Regions, the ISS and the CIRI, the participating general medicine and pediatric physicians, and university reference laboratories, under the coordination of the Ministry of Health.

 

Surveillance during the current influenza season began the 42nd week of 2000. As shown in Figure 1, which presents the weekly data available through the 9th week of 2001, the incidence remained low until the 3rd week of 2001, with values below 2/1000 enrolled patients. Beginning in the 4th week, however, there was a slight increase, and in the 6th week, the incidence reached its peak at 5.6/1000. Temporal patterns in incidence rates were similar for the three age groups, although the rates differed between the three. The highest incidence for all groups was during the 6th week of 2001, but the peak levels were 22/1000 for the 0-14 year age group, 5.0/1000 for those 15-64 years and 1.7/1000 for those ≥65 years.

 

Even though the influenza season is not yet finished and surveillance will continue until the 15th of April, it is evident that the epidemic this year was less intense than the previous year, when the incidence reached 13/1000. Furthermore, the peak in 2000 occurred during the second week of the year, four weeks earlier than the 2001 peak. The estimated number of cases in Italy in 2000-2001 will reach 2.4 million, less than half the number reported the previous year.

 

In contrast to the 1999-2000 season, the spread of the epidemic did not follow a geographic gradient from north to south. Similar to the 1999-2000 season, however, the highest incidence was in the youngest age group, which is the most susceptible by virtue of lack of exposure to previous influenza epidemics and because few are vaccinated. It should be noted that while rates in the older age groups were low this year, the 0-14 year age group had rates that were higher than those observed in 1999-2000.

 

Virologic surveillance has demonstrated that more than 90% of the strains circulating this year are Type A. Most (98%) of the Type A strains belonged to subtype A/H1N1. This differs from the previous year, when the predominant subtype was A/H3N2.

 

 The tendency for the influenza virus to mutate continually results in epidemic and pandemic cycles and makes it difficult to predict trends in the illness. For this reason, therefore, the low incidence of influenza this year should not lead a sense of false security. It is important that persons at risk continue to be vaccinated every year, given that the complications of the illness may result in increased mortality, especially among the elderly.

Figure 1. Incidence of Influenza-Like Syndrome by Age, Italy, 1999-2000 and 2000-2001.

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