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from december 2004 through november 2005 , a total of 651 fecal specimens were collected within 48 hours of symptom onset from 627 patients ( 43.5% male , < 197 years of age , 26.9% < 16 years of age ) with symptoms of gastroenteritis at prince of wales hospital , hong kong special administrative region , people 's republic of china .
local monthly mean air temperature during the study period was obtained from the hong kong observatory ( available from http://www.hko.gov.hk/wxinfo/pastwx/ywx.htm ) .
viral rna was purified from fecal specimens and transcribed to cdna as described ( 4 ) .
all specimens had a detectable level of human -actin cdna , which suggests high rna integrity .
nov gi and gii were detected by a quantitative and genogroup - specific real - time pcr assay , as previously described ( 5 ) .
three amplicons from each genogroup were directly sequenced to confirm their identities and genogroups on a 3100 genetic analyzer ( applied biosystems , foster city , ca , usa ) .
cdna viral load was quantified in triplicate per run against 10-fold serial dilutions ( 1010 copies ) of external plasmid standards prepared by cloning genogroup - specific amplicons into vector pcr2.1-topo ( invitrogen corp . ,
the lower detection limit of the assay was equivalent to 210 copies of cdna per gram of fecal specimen .
coefficients of variation within and between runs were calculated as the percentage of the ratio between the standard deviation and mean of threshold cycle numbers from the standard curves .
the respective intra- and interassay coefficients of variation for nov gi were 0%4.1% and 1.9%5.8% , respectively , and 0.1%6.1% and 2.7%6.6% , respectively , for nov gii .
two common gastroenteritis - associated viruses , sapovirus and group a rotavirus , were detected in parallel , as previously described ( 4,6 ) .
/ g1skr and g2fb / g2skr for gi and gii , respectively , as described elsewhere ( 7 ) .
since g1ff / g1skr and g2fb / g2skr completely spanned the region amplified by the rt - pcr assay , the 2 primer sets were also used to check for sequence complementarity among target , primers , and probe used in quantitation .
statistical analyses were performed by spss version 11.5.1 ( spss inc . , chicago , il , usa ) , and figures were constructed by prism version 4.03 ( graphpad software , inc . , san diego , ca , usa ) and spss .
novs were detected in 54 ( 8.3% ) fecal specimens . among the nov - positive specimens , 8 ( 14.8% )
were infected with gi , 37 ( 68.5% ) with gii , and 9 ( 16.7% ) were coinfected with gi and gii .
moreover , 3 ( 5.6% ) specimens were coinfected with sapovirus , 2 ( 3.7% ) with group a rotavirus , and 1 ( 1.9% ) with sapovirus and group a rotavirus .
the mean age of patients infected with nov gi and gii was 54.6 and 33.0 years , respectively ( p = 0.02 ) .
sex and hospitalization rates between patients infected with the 2 genogroups did not differ significantly ( table ) .
the median cdna viral load of nov gi and gii detected in the fecal specimens was 8.410 ( range 2.2102.910 ) and 3.010 ( range 2.5107.710 ) copies per gram of fecal specimen ( figure 1a ) , respectively .
although the range was comparable between the genogroups , the median of nov gii was > 100-fold higher than that of gi ( p = 0.0022 , 2-tailed mann - whitney u test ) .
similar findings were obtained when nov gi / gii coinfections ( p = 0.0066 , figure 1b ) or all viral coinfections ( p = 0.0042 , figure 1c ) were excluded .
seven of 9 specimens with nov gi / gii coinfections had higher cdna viral load of gii than gi , with fold changes from 4 to 452 ( median 248 ) ( figure 2 ) . furthermore , while nov was detected year - round , a marked seasonal trend was evident : higher prevalence occurred in winter months , when the cdna viral load of gii was generally higher compared with gi ( figure a1 ) .
scatterplots for cdna viral load of noroviruses ( nov ) genogroup i ( gi ) and gii .
c ) all positive isolates , excluding all those with viral coinfection ( nov gi and gii together with sapovirus , group a rotavirus , or both ) .
cdna viral load in 9 specimens with norovirus ( nov ) genogroup i ( gi ) and gii coinfection .
multivariate linear regression model was used to determine the potential association between cdna viral load , nov genogroup , and patient 's age .
after age stratification , the cdna viral load in fecal specimen was still significantly associated with nov genogroup (  = 0.390 , p = 0.002 ) .
however , no significant association was found between cdna viral load and age of patients (  = -0.060 , p = 0.626 ) .
of the 63 nov isolates , 43 ( 68.3% ) were successfully sequenced for phylogenetic analysis , including 7 gi and 36 gii isolates .
nov gi isolates covered at least 5 genotypes , but no circulating strain predominated ( figure a2 ) . for nov
gii isolates , we found > 8 genotypes ; gii/4 was the most prevalent ( figure a3 ) . to rule out the possibility of a quantitation artifact due to different stability between genogroups upon storage or freeze - thaw cycle , viral rna was re - extracted and requantitated from a fecal specimen that had been stored for > 6 months and coinfected with both nov gi and gii .
repeat testing showed no drop in cdna viral load for either genogroup , which suggests a comparable stability upon storage .
sequence complementarity among target , primers , and probe used in quantitation was also verified . while no sequence mismatch in primers and probe was found among the 7 nov gi isolates , 5 of the 36 gii isolates had a single mismatch .
thus , the low cdna viral load of nov gi measured was unlikely due to sequence mismatching .
in this study , we show for the first time that the median cdna viral load of nov gii is > 100-fold higher than that of gi in the fecal specimens of patients with nov - associated gastroenteritis . neither the possibility of quantitation artifacts as a result of primers and probe mismatching nor stability differences between genogroups on storage was likely to account for our observation .
moreover , 7 of 9 specimens with nov gi / gii coinfection exhibited higher cdna viral load of gii than that of gi . also , the cdna viral load of nov gii was usually higher than that of gi for each collection month , a finding that further supports our interpretation .
we speculate that the increased cdna viral load facilitates the transmission of nov gii from infected persons to susceptible hosts through the fecal - oral route .
studies on other viruses have shown that viral load correlates well with the epidemiology of diseases .
for example , the predominance of hiv-1 over hiv-2 has been suggested to be attributed to the higher viral load of hiv-1 ( 8) .
however , the implication of viral shedding pattern and cdna viral load on epidemiologic characteristics and clinical manifestations of novs deserves further investigation .
our findings provide strong molecular evidence for the worldwide predominance of nov gii and may open new research directions in the epidemiologic study of novs .
we report the median cdna viral load of norovirus genogroup ii is > 100-fold higher than that of genogroup i in the fecal specimens of patients with norovirus - associated gastroenteritis . 
 we speculate that increased cdna viral load accounts for the higher transmissibility of genogroup ii strains through the fecal - oral route .