Farmer
Senior Moderator
From ProMed mail, May 29, 2009
Transmission rate of influenza A (H1N1) 2009 infection calculated from a
school-based outbreak
- -----------------------------------------------------------------
A key determinant of the success of influenza containment is the
transmission rate of the novel strain. Fraser et al (1) estimated the basic
reproduction number (R0) of the Mexican outbreak of influenza A (H1N1) to
be in the range of 1.4-1.6. R0 is a key measure of transmissibility and
estimates the number of secondary cases in a completely susceptible
population. Their findings were comparable to lower estimates for the 1918
pandemic, where R0 ranged from 2-3 (2).
To further investigate the transmissibility of this novel virus we
conducted a secondary analysis of the largest reported cluster of influenza
A (H1N1) (3). Survey data from students of the St Francis Preparatory
School outbreak in the United States were used to calculate the outbreak
effective reproduction number (R) in a school-based setting. R is the
average number of secondary cases generated by an infectious case during an
epidemic and is comparable to R0. This survey collected data on
self-reported ILI (influenza-like illness -- fever AND either cough or sore
throat) between 8 Apr 2009 and 28 Apr 2009.
We used the method proposed by Vynncky et al (4), to calculate R using the
growth rate of the epidemic. Parameter assumptions were based on estimates
for seasonal influenza commonly reported in the literature, as the values
for this novel virus are not yet available. These were as follows:
incubation period of 2 days; infectious period of 3 days; and a calculated
serial interval of 5 days. The serial interval is the time between the
onset of symptoms for 1st and 2nd generation cases. Using daily data from
the outbreak growth phase, R was calculated at 2.69 (95 per cent, CI
2.20-3.22). Increasing the estimated infectious period to 5 days results in
an R of 3.45 (95 per cent, CI 2.74-4.28). The confidence interval [CI] for
R was derived from a Monte Carlo simulation based on the uncertainty of the
slope estimate. Estimates of R were relatively insensitive to the use of
data from the growth phase or entire outbreak.
Our calculated R is specific to this school setting and transmission rates
in the community are likely to be lower (2). The use of parameters
estimated from seasonal influenza will need confirmation for the 2009
influenza A H1N1 virus. Our analysis supports the findings from Fraser et
al (1) that this H1N1 virus has a transmission rate comparable to the lower
R0 estimates of the 1918 pandemic.
References
- ----------
1. C Fraser, et al. Pandemic potential of a strain of influenza A (H1N1):
early findings. [Published online May 14 2009; 10.1126/science.1176062
(Science Express Reports); available from
<http://www.sciencemag.org/cgi/rapidpdf/1176062v1.pdf>].
2. G Chowell, H Nishiura, L Bettencourt. Comparative estimation of the
reproduction number for pandemic influenza from daily case notification
data. J R Soc Interface 22 Feb 2007; 4(12): 155-66; doi:
10.1098/rsif.2006.0161 [available from
<http://rsif.royalsocietypublishing.org/content/4/12/155.full>].
3. New York City Department of Health and Mental Hygiene - St Francis Prep
Update: Swine Flu Outbreak. Available from
<http://www.nyc.gov/html/doh/downloads/pdf/cd/h1n1_stfrancis_survey.pdf>
(30 Apr 2009, accessed 5 May 2009).
4. E Vynncky, A Trindall, P Mangtani: Estimates of the reproduction numbers
of Spanish influenza using morbidity data. Int J Epidemiol 2007; 36: 881-9;
doi:10.1093/ije/dym071 [available from
<http://ije.oxfordjournals.org/cgi/content/full/36/4/881>].
- --
Bev Paterson (MAE), David N Durrheim (MD, DrPH), Frank Tuyl (PhD)
Hunter New England Area Health Service
Australia
and
Bev Paterson
Epidemiologist
Hunter New England Area Health Service
<bev.paterson@hnehealth.nsw.gov.au>
Transmission rate of influenza A (H1N1) 2009 infection calculated from a
school-based outbreak
- -----------------------------------------------------------------
A key determinant of the success of influenza containment is the
transmission rate of the novel strain. Fraser et al (1) estimated the basic
reproduction number (R0) of the Mexican outbreak of influenza A (H1N1) to
be in the range of 1.4-1.6. R0 is a key measure of transmissibility and
estimates the number of secondary cases in a completely susceptible
population. Their findings were comparable to lower estimates for the 1918
pandemic, where R0 ranged from 2-3 (2).
To further investigate the transmissibility of this novel virus we
conducted a secondary analysis of the largest reported cluster of influenza
A (H1N1) (3). Survey data from students of the St Francis Preparatory
School outbreak in the United States were used to calculate the outbreak
effective reproduction number (R) in a school-based setting. R is the
average number of secondary cases generated by an infectious case during an
epidemic and is comparable to R0. This survey collected data on
self-reported ILI (influenza-like illness -- fever AND either cough or sore
throat) between 8 Apr 2009 and 28 Apr 2009.
We used the method proposed by Vynncky et al (4), to calculate R using the
growth rate of the epidemic. Parameter assumptions were based on estimates
for seasonal influenza commonly reported in the literature, as the values
for this novel virus are not yet available. These were as follows:
incubation period of 2 days; infectious period of 3 days; and a calculated
serial interval of 5 days. The serial interval is the time between the
onset of symptoms for 1st and 2nd generation cases. Using daily data from
the outbreak growth phase, R was calculated at 2.69 (95 per cent, CI
2.20-3.22). Increasing the estimated infectious period to 5 days results in
an R of 3.45 (95 per cent, CI 2.74-4.28). The confidence interval [CI] for
R was derived from a Monte Carlo simulation based on the uncertainty of the
slope estimate. Estimates of R were relatively insensitive to the use of
data from the growth phase or entire outbreak.
Our calculated R is specific to this school setting and transmission rates
in the community are likely to be lower (2). The use of parameters
estimated from seasonal influenza will need confirmation for the 2009
influenza A H1N1 virus. Our analysis supports the findings from Fraser et
al (1) that this H1N1 virus has a transmission rate comparable to the lower
R0 estimates of the 1918 pandemic.
References
- ----------
1. C Fraser, et al. Pandemic potential of a strain of influenza A (H1N1):
early findings. [Published online May 14 2009; 10.1126/science.1176062
(Science Express Reports); available from
<http://www.sciencemag.org/cgi/rapidpdf/1176062v1.pdf>].
2. G Chowell, H Nishiura, L Bettencourt. Comparative estimation of the
reproduction number for pandemic influenza from daily case notification
data. J R Soc Interface 22 Feb 2007; 4(12): 155-66; doi:
10.1098/rsif.2006.0161 [available from
<http://rsif.royalsocietypublishing.org/content/4/12/155.full>].
3. New York City Department of Health and Mental Hygiene - St Francis Prep
Update: Swine Flu Outbreak. Available from
<http://www.nyc.gov/html/doh/downloads/pdf/cd/h1n1_stfrancis_survey.pdf>
(30 Apr 2009, accessed 5 May 2009).
4. E Vynncky, A Trindall, P Mangtani: Estimates of the reproduction numbers
of Spanish influenza using morbidity data. Int J Epidemiol 2007; 36: 881-9;
doi:10.1093/ije/dym071 [available from
<http://ije.oxfordjournals.org/cgi/content/full/36/4/881>].
- --
Bev Paterson (MAE), David N Durrheim (MD, DrPH), Frank Tuyl (PhD)
Hunter New England Area Health Service
Australia
and
Bev Paterson
Epidemiologist
Hunter New England Area Health Service
<bev.paterson@hnehealth.nsw.gov.au>
