Although the average incidence of measles in China decreased in 2006-2007 compared with 2005, however measles incidence was >10/100,000 in 8 and 9 developed provinces in 2006 and 2007 respectively (Figure 1), reaching the goal of measles elimination by 2012 will be a challenge. Two reasons may account for the recent increase in measles cases in China. Over 30% of measles cases from 2005 to 2007 in China were in the "floating population" (persons without a permanent residence card for the place in which they live) that account for up to 50% of the population in some developed provinces and big cities . The floating population is not able to access routine immunization services outside their official residential area and is likely to constitute a large proportion of the immunity gap in China. The proportion of floating people has increased since 2005 following migration from rural based developing provinces to the more urban based, developed provinces. A high proportion of measles cases in young adults in cities may be responsible for the measles incidence increase among the developed provinces from 2005 to 2007 as this group received only a single dose of measles vaccine as infants.
With the attainment of Universal Childhood Immunization goals , measles morbidity in China reached lows in 1995. During 1995-2004, the incidence of measles was approximately 6/100,000 population every year. There was a resurgence of measles cases in 2005 in China where the incidence increased to 10/100,000 population. In 2006, the MOH issued an activity plan to achieve measles elimination by 2012. The plan recommended that the age of the first dose of measles-containing vaccine (MCV1) remain at 8 months, and the age of the second dose of measles-containing vaccine (MCV2) change from 7 years to 18-24 months. China's reported two dose MCV coverage ranged from 84.1% to 96.4% during 2003-2007, with a mean of 92.5% . The incidence dropped to 6.8/100,000 in 2006 and 7.2/100,000 in 2007, because of the measles supplementary immunization activities (SIAs) in some high incidence provinces. In China, SIAs during 2003-2008 reached approximately 101 million children and adolescents in 14 provinces . The age-specific incidence of measles cases was highest among young children and declined with increasing age from 2006-2007. However, the incidence rate had 2 peaks, in infants (< 1 year old), and in adolescents and adults (>15 years old), compared to 1995-2004. Although the developed provinces had different incidence rates, there were no major differences in the age distribution of cases. Furthermore, the change to the measles vaccine schedule, in which the second dose administered to children was changed from 7 year old to 18-24 months in 2005, could have accounted for changing the age distribution.
The virologic surveillance for measles virus that began in China in 1993 is one of the most thorough programs in the global network. Through this time only a single genotype, H1, has been detected in China. Measles surveillance in other countries in Asia has confirmed that endemic circulation of genotype H1 is limited to only China . Analyses conducted during the early part of the last decade identified two lineages within the Chinese H1 viruses and these were designated cluster 1 and cluster 2 with cluster 1 viruses being the most frequently detected strains. The genetic analysis of wild-type measles isolated in 2006 and 2007 indicated that cluster 1 is still the most frequently detected lineage in China. There were no cluster 2 viruses isolated during 2006-2007, compared with 26.9% of all strains detected in 1993-2000 and 5.4% in 2001-2005 [12, 13, 15, 16]. This finding suggested that measles vaccination programme was able to reduce the genetic diversity of circulating measles virus from 1993 to 2007 in China. Even though the genotype H1 viruses in China are genetically diverse, no geographic restriction of particular lineages is evident. The major neutralization sites and N-glycosylation sites in the hemagglutinin were not changed in the more recent genotype H1 viruses compared to those isolates. Compared China vaccine strain, the 719th nucleotide of H1 wild strains in China was changed from G to A in amino acid mutation from Ser to Asn. Furthermore, one N-glycosylation site in 240th amino acids was loosen because of the mutation . Therefore, the amino acid mutation in the H protein of the Chinese viruses did not appear to the result in loss of mayor neutralization epitopes by cross antibody induced following vaccination.
MeVs with identical sequences were associated with outbreaks in multiple provinces, and these lineages were detected in multiple provinces for several years. The sequences from the recent genotype H1 viruses that were imported into Europe, Taiwan and Hong Kong showed 100% sequence identity with genotype H1 viruses that were circulating in mainland China or were associated with international importation of virus . Molecular epidemiologic studies of measles virus are an important component of measles surveillance in China because they will help to document interruption of transmission of the endemic genotype, H1, a key criterion for verification of measles elimination. Measles surveillance is also necessary to monitor the stability of the neutralization epitopes on the viral hemagglutinin.