South-East Asia Regional Conference on Epidemiology

140 | <strong>South</strong>-<strong>East</strong> <strong>Asia</strong> <strong>Regional</strong> <strong>Conference</strong> on Epidemiology
Fig. 3: Modelling the HIV epidemic in a SF MSM cohort
On the basis of an underlying logical framework and mathematical models, UNAIDS has correctly
concluded that the AIDS programmes have contributed to a reduction in HIV incidence and AIDS
mortality. However, UNAIDS has also cautioned that before a specific attribution can be made, an
understanding of the dynamics of HIV epidemics and the variety of factors that contribute to HIV
transmission and AIDS mortality other than HIV interventions is required to adjust for potentially
confounding and contributory factors(6).
Influenza
Evaluating the effectiveness of influenza prevention/control programmes is even more challenging
than evaluating the success of HIV/AIDS programmes since new influenza viruses pose a constantly
changing and moving target for prevention programmes. A new potentially pandemic influenza virus
can emerge suddenly but the severity or potential impact of any new influenza virus can only be
measured as it spreads through populations. All influenza viruses are different with regard to their (1)
ability to spread in human populations; (2) antigenic differences and similarities to previous seasonal
or pandemic viruses; (3) clinical spectrum of disease, and (4) mortality rate. (7-9)
Estimates of influenza case-fatality rates (CFR) are difficult to use to compare the severity of
influenza viruses because the definition of a ‘case’ of influenza has not been well defined and can
range from all infections to only those infected persons who are hospitalized. Estimates of influenza
mortality rates, in a defined population, of pandemic and recent ‘seasonal’ influenza viruses can
provide a reasonably objective yardstick to assess the relative severity of any new influenza virus.
However, at the outset or beginning of a new influenza pandemic, the severity of any new influenza
virus is unknown. This makes it virtually impossible to estimate what impact, if any, public health
interventions, whose effectiveness are also unknown, may have on a new influenza pandemic.