The epidemiology of norovirus in England diagnostics, incidence and transmission
Existing estimates of the incidence of 
infectious intestinal 
disease 
(IID) 
caused 
by 
norovirus in the community are based on 
electron 
microscopy 
or 
reverse 
transcription- 
polymerase chain reaction (RT-PCR). 
Neither 
method 
accurately 
represents 
norovirus 
disease burden: electron microscopy 
has 
poor 
diagnostic 
sensitivity, 
RT-PCR 
has 
poor 
diagnostic specificity. 
In this thesis, for the first time, viral 
load 
measurements 
were 
used 
to identify 
cases of 
norovirus-associated IID, to update 
the 
existing, 
electron 
microscopy-based 
estimates 
of norovirus disease burden in the community 
in England 
and 
to 
conduct a 
comprehensive analysis of risk factors for 
sporadic 
norovirus-associated 
IID. The 
prevalence and characteristics of asymptomatic norovirus 
infection in 
England 
were 
also described. Data for this work were 
taken from the 
Study 
of 
Infectious Intestinal 
Disease faecal specimen archive, which 
had been 
subject 
to 
semi-quantitative real- 
time RT-PCR norovirus testing. Finally, routine surveillance 
data 
and 
time-series 
regression modelling were used 
to 
produce an 
additional and 
independent 
estimate 
of 
the incidence of general practice consultations 
for 
norovirus-associated 
IID in 
England 
and Wales. 
Using viral load for norovirus diagnosis, the 
age-adjusted community 
incidence 
of 
norovirus-associated IID in England was 
4.5 
per 
100 
person-years 
(95% 
credibility 
interval: 3.8,5.2), equating to 2 million episodes per 
year 
between 
1993 
and 
1996. 
Amongst children aged less than five 
years, 
the 
community 
incidence 
was 
21.4 
per 
100 
person-years (95% credibility interval: 15.9,27.7) 
and 
the incidence 
of consultations 
to 
general practitioners for norovirus-associated 
IID 
was 
3.2 
per 
100 
person-years 
(95% 
credibility interval: 2.6,3.8), with 
100 000 
children 
visiting 
their 
GP for 
norovirus- 
associated IID each year. 
The main risk factor for sporadic, community-acquired 
norovirus-associated 
IID 
was 
contact with a person with IID symptoms. 
This 
result 
indicates that 
reduction 
of person- 
to-person transmission would substantially 
decrease the 
burden 
of norovirus- 
associated IID in the community 
in England, 
e. g. 
through 
good 
hand hygiene 
and 
appropriate cleaning of environmental surfaces.