Introduction
Measles is one of the most infectious diseases known to man and remains the leading cause of vaccine preventable deaths in children worldwide. In many European countries measles is still a cause of great public health concern (1-3). Despite the easy availability of vaccination for its control, vaccination uptake has been sub-optimal in some countries (4,5). However, following the successful interruption of indigenous wild poliovirus transmission in the European Region, many countries in the region are eager to move towards targeting measles elimination and strengthening measles surveillance, and have started implementing elimination strategies.
The commencement of measles surveillance in EUVAC.NET participating countries heralded the network entering into its implementation phase and an important step in the early stage of the measles elimination programme (6). The following surveillance report covers the years 2001 and 2002 and aims to describe basic epidemiological features of measles in EUVAC.NET participating countries. These include all 15 Member states of the EU together with Iceland, Norway, Malta, and Switzerland.

Methods
The case definition used for identifying a case of measles follows the WHO recommendations (7). Its application varies to some degree in the various national surveillance systems depending on the stage of control of indigenous measles (6). Data from participating countries were sent electronically to EUVAC.NET as an Excel file. Analysis of data was carried out using Microsoft Excel 2000. In this report, evaluation is based on cases with disease onset dates in the corresponding years. Where disease onset dates were not available or considered invalid, the date of notification was used instead. In some countries minor discrepancies with national data may arise if these include cases notified in 2001 and 2002 but with onset dates in a preceding year. Incidence rates were based on population statistics obtained from the Population Information page on the WHO website for the Computerized Information System for Infectious Diseases (CISID), http://cisid.who.dk. Epidemiological evaluation of data from sentinel surveillance systems is included separately. All incidence rates are based on reported measles cases and are per 105 population per year. With a few exceptions, total numbers reported are likely to be incomplete. This report contains incidence data that have to be interpreted cautiously because of under-reporting.

Results
Participation
For 2001, all 19 EUVAC.NET participating countries provided measles surveillance data. Sixteen countries operating mandatory notification systems for measles provided case-based data. In addition to data from such notification systems, Germany and Switzerland also provided data from sentinel surveillance systems.
In Germany, around 1 100 physicians, mostly paediatricians amounting to 15% of all paediatricians in the country, participated in such a system. The Swiss sentinel surveillance data involved the participation of 3-4% of all primary care physicians. In Austria and France (4), sentinel systems are the only surveillance systems in place for reporting measles cases. The Austrian system involved 226 physicians and covered 8% of the population. In France, the sentinel surveillance system involved 300 general practitioners. Voluntary reporting took place in the French community of Belgium that provided an annual figure for measles cases.
For 2002, 17 (89%) of the EUVAC.NET countries provided data, 16 of which contributed case-based data obtained through mandatory notification systems. Switzerland also provided data based on a sentinel surveillance system that involved the same proportion of physician participation as in 2001. As in 2001, the French data provided was derived from the sentinel surveillance system. It involved the participation of 472 general practitioners.

Incidence - notifications and laboratory data
For 2001 and 2002, EUVAC.NET gatekeepers reported a total of 7 428 and 10 500 measles cases giving a crude incidence of 2.36 and 3.37 per 100 000 population respectively (Table 1). Confirmed cases, either by laboratory methods or those epidemiologically confirmed constituted 44% and 27% of the total cases for the corresponding years. Of these, 30% and 44% were laboratory-confirmed for the successive years.

The distribution of notified measles cases varied considerably among the participating countries. In Figure 1, reporting countries have been grouped into low, moderate and high incidences for notified cases in 2001 and 2002. In 2001, no cases were reported from Iceland. The highest incidence of measles notifications was reported from Germany. In 2002, no cases were reported from Finland, Iceland and Luxembourg while the highest incidence of measles notifications was reported from Italy.



Outbreaks and imported cases
For 2001 and 2002, 18% and 20% of reported cases respectively were outbreak-related, i.e. occurred in outbreak situations where localised increases in the incidence of measles were identified (7). Most were reported from Germany (1) (95% in 2001 and 83% in 2002). No link to an outbreak could be made in 17% of cases in 2001 and 51% in 2002. No data from Italy were available on outbreak related cases through the mandatory notification system. Nevertheless, a paediatric sentinel surveillance system has monitored a large outbreak which took place mostly during the first half of 2002 (2,3).
A known link to an imported case was reported in 70% and 86% of cases in 2001 and 2002 respectively. Of these, there were 37 and 84 imported cases for the consecutive years amounting to 0.7% and 0.9% of all cases with known link to an imported case respectively. The rest were cases believed to have been infected within their own country. In 2001, eight cases (22%) were imported from another European country namely Belgium, France, Germany, Spain and the United Kingdom (UK). Fifty-one percent of the imported cases were from Asia. In 2002, there were 52 (62%) imported cases from within Europe followed by Asia with 20 (24%) cases. Italy was implicated as the source of infection in 40% of all importations within Europe followed by Austria (8%) and Germany (6%).

Age distribution and seasonality
In 2001 and 2002, measles was reported in both children and adults with the 1-9 year age group (Figure 2) accounting for 52% and 65% of the cases respectively. The proportion of laboratory confirmed cases generally increased with increasing age group (Figure 2). The majority of cases were reported in the first half of the year (75% in 2001 and 90% in 2002) (Figure 3).

Vaccination status
Information on vaccination status was provided in 56% and 72% of the total measles cases in 2001 and 2002 respectively. In 2001, half of the reported measles cases occurred in unvaccinated children (table 2). This proportion increased to 66% in 2002.

Morbidity and mortality
Data on known hospitalisation status increased from 69% in 2001 to 89% in 2002. In 2001, there were 278 cases that were hospitalised in connection with measles representing 5% of all cases with known hospitalisation status. Most cases were reported from Germany (44%) and Italy (40%). In 2002, hospitalised cases amounted to 822 representing 9% of all cases with known hospitalisation status. Of these, 83% were reported from Italy.
In 2001, encephalitis as a severe complication of measles was reported in 21 cases giving an incidence of 283 per 100 000 measles cases. All were reported from Germany. Most cases (71%) were in the 1-9 year age group, four cases were in older age groups and in one case the age was unknown. Twelve cases were unvaccinated, three cases had a history of vaccination and in the remaining six cases the vaccination status was unknown. One death directly attributed to measles in an 8-month-old male was also reported from Germany.
Encephalitis was reported in six cases in 2002 giving an incidence of 57 per 100 000 measles cases. Five cases were reported from Germany and one from Switzerland. They were aged 8, 12, 16, 19, 24, and 39 years. Three cases were unvaccinated and in the other three the vaccination status was unknown. A case of subacute sclerosing panencephalitis in a six-year-old male was identified in Norway in 2002. The child is believed to have suffered from measles in his country of origin before being adopted in Norway. This case was not included as a measles case in the EUVAC.NET database. There were no deaths reported in 2002. However, in the UK, measles may have been a contributory factor in the death of an adult with severe lung disease who was also confirmed as having the infection (8).

Data from Sentinel Surveillance Systems
For 2001, Austria reported 13 cases giving an extrapolated crude incidence of 2.0 per 100 000 population. Germany reported 878 cases with an extrapolated crude incidence rate of about 14 per 100 000 population. Both France and Switzerland reported 22 cases each, extrapolated to 8 460 and 700 cases respectively. Based on these figures the estimated crude incidence rates are 12 per 100 000 population for France (4) and 9.77 per 100 000 for Switzerland. For 2002, the Swiss sentinel system reported an extrapolated 500 cases and a crude incidence rate of 7 per 100 000 population. The French sentinel system reported 12 cases giving an extrapolated 5 190 cases and an incidence of 8 per 100 000 population. The above estimates were based on population figures obtained by national surveillance centres.

Discussion
Most participating countries have provided case-based measles data derived from routine notification systems. Whilst in the greater part of participating countries notification is statutory (9) and universal case reporting is widely used for surveillance, under-reporting is common and must be taken into account when interpreting data. Some participating countries have sentinel surveillance systems in place. Where the incidence of disease is relatively high, such systems may even be more accurate than a deficient universal notification system. However, as the disease incidence declines, estimates using sentinel surveillance systems become less valid and therefore less suitable for surveillance purposes in the elimination phase.
The findings in this report document a widely varied incidence rate for measles in the different EUVAC.NET participating countries. While some countries have reported incidence rates of less than 0.1 per 100 000 population per year, indicating a near-elimination situation, the disease is still endemic in other countries. Compared to 2001, there was a 41% increase in the case-based reported measles cases in 2002. While reports of measles cases from Germany dropped by 23%, cases reported from Italy increased six fold, greatly contributing to the increase in the total number of cases.

Although no relationship with vaccination coverage for measles is attempted in this report, it is assumed that differences in incidences are due to varying degrees of success in vaccination programmes. Overall, 66% of those reported with measles in 2002 were unvaccinated compared with 50% in the previous year. Paradoxically, a large proportion of measles occurring in unvaccinated children was reported from countries known to have moderate-to-high vaccination coverage such as Denmark (10), Norway (11), and The Netherlands (12). This is due to an accumulated cohort of susceptible individuals to the disease (ie no history of measles or vaccination) over time following the introduction of MMR vaccination in the national vaccination programme. This is known to occur during the so called 'honeymoon period'; the name given when the very low incidence currently suggests that a disease is disappearing, yet without any sign, groups of unprotected individuals are forming the source of possible future outbreaks. This becomes particularly relevant when pockets of unvaccinated individuals accumulate.
Many participating countries are regularly reporting data to EUVAC.NET and the quality of data on the status of importation, vaccination and hospitalisation has improved in 2002. However, others still need to have regular reporting and improvement of data quality in place. The importance of strengthening surveillance systems by vigorous case investigation forms part of the strategy to interrupt the indigenous transmission of measles within Europe. Apart from the collection of additional data on measles cases reported under routine surveillance, enhanced surveillance also requires timely national case-based reporting on at least a monthly basis. In addition, as vaccination coverage increases and measles reaches a near-elimination phase in Europe, it becomes increasingly important to investigate suspected cases. Hence, rapid laboratory confirmation is an essential tool of enhanced surveillance. Such high quality surveillance is crucial to monitor trends, identify the target population for vaccination programmes and determine whether coverage objectives are being reached. EUVAC.NET is committed to facilitate the improvement of measles surveillance and together with its future plans to collect data on vaccination coverage it will be contributing to the elimination of measles within the region by 2010.

Remerciements / Acknowledgements
Nous remercions tous les correspondants nationaux de EUVAC.NET qui ont contribué à la collecte des données de ce réseau de surveillance. / We would like to thank all EUVAC.NET gatekeepers and reporters who have contributed data to this surveillance network:
Reinhild Strauss, Federal Ministry for Health and Women, Austria; Tinne Lernout, Scientific Institute of Public Health, Belgium; Irja Davidkin, National Public Health Institute, Finland; Isabel Bonmarin, Institut de la Veille Sanitaire, France; Anette Siedler, Robert Koch-Institut, Germany; Takis Panagiotopoulos, National Centre for Surveillance and Intervention, Greece; Gudrún Sigmundsdóttir, Directorate of Health, Iceland; Darina O'Flanagan, Margaret Fitzgerald and Sarah Gee, National Disease Surveillance Centre, Ireland; Loredana Vellucci, Ministry of Health, Italy; Pierrette Huberty-Krau, Direction de la Santé, Luxembourg; Andrew Amato Gauci and Jackie Maistre Melillo, Health Division, Malta; Susan van den Hof and Hester de Melker, National Institute of Public Health and the Environment (RIVM), The Netherlands; Øistein Løvoll, National Institute of Public Health, Norway; Maria Da Graça Gregório de Freitas and Helena Alves Pereira, National Institute of Health, Portugal; Carmen Amela, Instituto de Salud Carlos III, Spain; Hans Blystad, Swedish Institute for Infectious Disease Control, Sweden; Jean-Luc Richard, Swiss Federal Office of Public Health, Switzerland; Joanne White, Health Protection Agency, Communicable Disease Surveillance Centre, UK.