When animal viruses cross the species barrier to infect humans, few attain the capacity for human-to-human transmission. Only this latter framework that large epidemics with severe disease outcome ensue. This was evident in the 2003 severe acute respiratory syndrome (SARS) and ebola epidemic in West Africa in 2014-15. While the spread of these viruses in humans is aided by low or even non-existent herd immunity, the outcome of infection is thought to be driven by the lack of adaptation in humans. Consequently, it is possible that the rate of asymptomatic infection of newly emerged viruses is low. Indeed, this was the observation for infection with SARS coronavirus in 2003 where no asymptomatic infection was detected in seroprevalence studies in healthcare workers here in Singapore. Hence, case isolation, tracing of contacts with cases and quarantine were effective in stemming the spread of this virus in Singapore.
In 2009, a new strain of H1N1 emerged in the Americas and quickly swept across the world and Singapore was not spared. The Ministry of Health of Singapore initially adopted the same strategy of case isolation, contact tracing and quarantine to contain the spread of the H1N1 virus in Singapore. Unlike SARS, however, this strategy did not produce the desired results and the strategy was abandoned after the first several weeks of the epidemic in Singapore. One possibility for the failure of the “SARS strategy” to contain H1N1 is the difference in the asymptomatic infection rate, where individuals without overt clinical symptoms could continue to transmit the virus in the community. Hence, the first part of the thesis sought to determine the proportion of asymptomatic infection with pandemic influenza A H1N1 2009. Results from this thesis shows, through a prospective cohort study, that there is high proportion (79.6%) of asymptomatic infection for the newly emerged pandemic influenza A H1N1 2009. The high proportion of asymptomatic infection of the pandemic influenza A H1N1 2009 could explain the inefficiency of isolation and quarantine interventions to contain the spread of pandemic influenza A H1N1 2009.
For the second part of this thesis, the prevalence and temporal trends of respiratory pathogens in adults that presented with acute febrile illness at primary healthcare centers was investigated. Such epidemiological data could guide the implementation of vaccination programs which is especially relevant in tropical countries. This is especially pertinent as epidemic influenza activity is less seasonal in the tropics than in temperate countries. Data in this thesis shows that there is a strong bi-annual seasonal pattern for influenza A. Furthermore, the peak in the middle of the year (April – June) is higher than the peak at the end or beginning of each year (November - February). The findings in this thesis provide new perspective on prevention of influenza and other respiratory viruses in Singapore.
|School:||National University of Singapore (Singapore)|
|School Location:||Republic of Singapore|
|Source:||MAI 55/03M(E), Masters Abstracts International|
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