Studies beg question: Increased susceptibility to SOIV H1N1 infection in younger individuals?

INFLUENZA A (H1N1) - WORLDWIDE (54): DYNAMICS
*********************************************
A ProMED-mail post

ProMED-mail is a program of the
International Society for Infectious Diseases


Date: Mon 1 Jun 2009
From: David Fisman


A comment re: ProMED-mail Influenza A (H1N1) - worldwide (51): 20090529.1999
----------------------------------------------------------------------------
We read with interest the recent ProMED-mail posting by Bev Paterson
and colleagues [1]. These epidemiologists estimated the reproductive
number (R) for the novel swine-origin influenza A strain (SOIV H1N1)
that caused an outbreak in the St Francis Prep School in New York as
2.7 in that particular school setting. They note that another recent
estimate of R in the community setting, based on events in Mexico in
April 2009, was lower (approximately 1.5) [2].

One potential reason for the discrepancy in estimates of R in these 2
settings relates to the possibility that, notwithstanding the novel
nature of SOIV H1N1, older individuals in the population are
partially protected against infection by prior exposure to
antigenically similar viruses. This is consistent with the young age
of cases reported in Mexico, the United States, Canada, and Japan
[3,4]. Younger age distributions of cases were also noted in the 1918
and 1968 influenza pandemics [5,6], and have been attributed to
early-life exposure to related influenza strains in older individuals
[5].

If indeed older individuals are somewhat protected against infection
with this virus, it becomes easy to reconcile the gap between the 2 R
estimates described above. The R estimated in a young population
(that is, in a school outbreak) should approximate R0 (the
reproductive number in an entirely susceptible population) whereas
the R estimated by Fraser and colleagues would simply be an
"effective R" (Re) in a partially susceptible population.

The relationship between R0 and Re in a well-mixed population is
Re = R0 x S
where S is the proportion of the population susceptible to infection.

Re/R0 in this case would be approximately 0.55, or 55 percent of the
population susceptible. It is interesting to note that approximately
55 percent of the US population, for instance, is younger than age 40
[7]. Individuals older than 40 may have prior exposure to influenza
strains in circulation prior to the 1968 or 1957 pandemics, and such
exposure might confer a degree of protection against infection with
SOIV H1N1.

Increased susceptibility to SOIV H1N1 infection in younger
individuals, relative to older adults, would have several important
epidemiological implications: it would suggest that schools,
universities, military bases, and other areas with large
concentrations of younger individuals will be major foci for SOIV
influenza outbreaks; it would mean that optimal vaccination
strategies (should a vaccine become available) will target younger
adults than are usually targeted in seasonal influenza vaccination
campaigns [6]; and it would mean that low-income countries, which
have "triangular" population distributions (younger individuals
greatly in excess of older individuals) will have attack rates higher
than seen in countries with larger proportions of older individuals.

References
----------
1. ProMED-mail Influenza A (H1N1) - worldwide (51): dynamics
20090529.1999. 29 May 2009
2. Fraser C, Donnelly CA, Cauchemez S, et al: Pandemic Potential of a
Strain of Influenza A (H1N1): Early Findings. Science 2009. Available
from
. Last accessed
1 Jun 2009.
3. CDC: Novel Influenza A (H1N1) Virus Infections - Worldwide, May 6,
2009. MMWR Morb Mortal Wkly Rep 2009; 58(17): 453-8. Available from
.
4. Normile D: H1N1 Rocks Japan, But WHO Says It's Still No Pandemic.
ScienceInsider 2009. Available from
.
Last accessed 19 May 2009.
5. Ahmed R, Oldstone MB, Palese P: Protective immunity and
susceptibility to infectious diseases: lessons from the 1918
influenza pandemic. Nat Immunol 2007; 8(11): 1188-93. Abstract
available from
.
6. Miller MA, Viboud C, Olson DR, Grais RF, Rabaa MA, Simonsen L:
Prioritization of influenza pandemic vaccination to minimize years of
life lost. J Infect Dis 2008; 198(3): 305-11. Abstract available from
.
7. US Bureau of the Census: Population Estimates. Available from
.
Last accessed 31 May 2009.

--
Amy L Greer, PhD
Research Institute of the Hospital for Sick Children
Toronto, Ontario, Canada


Ashleigh Tuite, MHSc
Research Institute of the Hospital for Sick Children
Toronto, Ontario, Canada


Victoria Ng, BSc (hons), PhD (cand)
Research Institute of the Hospital for Sick Children
Toronto, Ontario, Canada
The Australian National University
Canberra, Australia


David N Fisman, MD MPH
Research Institute of the Hospital for Sick Children
Ontario Agency for Health Protection and Promotion
University of Toronto
Toronto, Ontario, Canada


[ProMED-mail welcomes this comment from David Fisman and colleagues
concerning the estimation of an "effective" R (Re) and its relevance
to vaccination policy should an appropriate SOIV H1N1 vaccine become
available. - Mod.CP]

[Relevant findings from a very recent CDC study were:
"...using stored serum specimens collected during previous vaccine
studies, CDC assessed the level of cross-reactive antibody to the
novel influenza A (H1N1) virus in cohorts of children and adults
before and after they had been
vaccinated with ... influenza season vaccines. The results indicated
that before vaccination, no cross-reactive antibody to the novel
influenza A (H1N1) virus existed among children. Among adults, before
vaccination, cross-reactive
antibody was detected in 6-9 percent of those aged 18-64 years and in
33 percent of those aged greater than 60 years" -- see: Influenza A
(H1N1) - worldwide (39) 20090521.1903. - Mod.JW]

[see also:
Influenza A (H1N1) - worldwide (53): case counts 20090531.2025
Influenza A (H1N1) - worldwide (52): seasonal vaccine 20090530.2010
Influenza A (H1N1) - worldwide (51): dynamics 20090529.1999
Influenza A (H1N1) - worldwide (50): swine immunity 20090528.1987
Influenza A (H1N1) - worldwide (40): case counts 20090521.1906
Influenza A (H1N1) - worldwide (30): case counts 20090516.1831
Influenza A (H1N1) - worldwide (20): case counts 20090510.1741
Influenza A (H1N1) - worldwide (10): case counts 20090504.1675
Influenza A (H1N1) - worldwide (02): case counts 20090430.1638
Influenza A (H1N1) - worldwide 20090430.1636
Influenza A (H1N1) "swine flu": worldwide (07), update, pandemic 5
20090429.1622
Influenza A (H1N1) "swine flu": Worldwide 20090427.1583
Influenza A (H1N1) virus, human: worldwide 20090426.1577
Influenza A (H1N1) virus, human - New Zealand, susp 20090426.1574
Influenza A (H1N1) virus, human - N America (04) 20090426.1569
Influenza A (H1N1) virus, human - N America 20090425.1552
Acute respiratory disease - Mexico, swine virus susp 20090424.1546
Influenza A (H1N1) virus, swine, human - USA (02): (CA, TX) 20090424.1541
Influenza A (H1N1) virus, swine, human - USA: (CA) 20090422.1516
Influenza A (H1N1) virus, swine, human - Spain 20090220.0715
2008
----
Influenza A (H1N1) virus, swine, human - USA (TX) 20081125.3715
2007
----
Influenza A (H2N3) virus, swine - USA 20071219.4079
Influenza, swine, human - USA (IA): November 2006 20070108.0077]
........................................cp/mj/jw
*##########################################################*
************************************************************
ProMED-mail makes every effort to verify the reports that
are posted, but the accuracy and completeness of the
information, and of any statements or opinions based
thereon, are not guaranteed. The reader assumes all risks in
using information posted or archived by ProMED-mail. ISID
and its associated service providers shall not be held
responsible for errors or omissions or held liable for any
damages incurred as a result of use or reliance upon posted
or archived material.
************************************************************
Become a ProMED-mail Premium Subscriber at

************************************************************
Visit ProMED-mail's web site at .
Send all items for posting to: promed@promedmail.org
(NOT to an individual moderator). If you do not give your
full name and affiliation, it may not be posted. Send
commands to subscribe/unsubscribe, get archives, help,
etc. to: majordomo@promedmail.org. For assistance from a
human being send mail to: owner-promed@promedmail.org.
############################################################
############################################################