Download On herd immunity and the 2006-chikungunya epidemic outbreak in

On herd immunity and the 2006-chikungunya epidemic
outbreak in Mauritius
Paper Presenter: Satish K Ramchurn, Department of Physics, Faculty of Science,
University of Mauritius
Author(s):
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Satish K Ramchurn, Department of Physics, Faculty of Science, University of
Mauritius
Smita SD Goorah, Department of Medicine, Faculty of Science, University of
Mauritius
Kavish Moheeput, Department of Physics, Faculty of Science, University of
Mauritius
Introduction
Herd immunity is a concept widely used in the epidemiology of viral infections to describe
the level of immunity to the infection in a population (herd) at which the virus stops
propagating epidemically. Should a vaccine to the infection exist, the herd immunity level
then indicates the proportion of the population that needs to be vaccinated so that the rest
of the population is safe from the disease. In terms of the basic reproductive number (R0),
i.e. the average number of secondary infections generated by a primary case entering a
totally susceptible population, the herd immunity level is (1 – 1/R0). Therefore, the herd
immunity level increases as R0 increases. Highly contagious infectious diseases have high
values of R0 and therefore a high vaccination coverage is required to control their
propagation.
During the months of February, March and April 2006, Mauritius (population of 1.2 million)
experienced a major epidemic outbreak of the mosquito-borne viral disease chikungunya.
It is believed that infection to chikungunya confers life-long immunity to the disease. This
disease became a global health concern following epidemic outbreaks in 2005/2006 in
Mauritius and in other south-west Indian Ocean islands such as Reunion, Mayotte and the
Comoros and in India affecting a total of about one million people in these countries. More
recently, a minor outbreak occurred in the Ravenna (Northern Italy) arousing concern that
chikungunya may establish itself in north temperate countries. The Mauritius 2006epidemic faded out towards the end of April and no case has been reported in the island
since September 2006.
Why has chikungunya not reappeared in Mauritius since September 2006? We have
investigated this query using a herd-immunity approach.
Methods
A compartmental human-mosquito interaction model was used to simulate the temporal
evolution of chikungunya. The human population was divided into a susceptible group, an
exposed group, an infectious group, and a group which had recovered from the disease
(SEIR model). The mosquito population was divided into a susceptible group, an exposed
group, and a group which was infectious. A set of differential equations was used to
describe the interaction of the groups. The equations were integrated computationally to
simulate the disease dynamics described by the differential equations for various initial
percentages of humans having acquired immunity as a result of having been affected in a
previous epidemic.
Results
It was found that the chikungunya herd immunity level corresponded to about 50% of the
population having been previously affected and acquired immunity from the disease.
Discussion
According to Mauritius Ministry of Health figures, there were 11,165 reported cases of
chikungunya in Mauritius in 2006. Although the actual number of cases was probably much
more, it is unlikely that any district of Mauritius experienced a chikungunya attack rate
nearing 50% except perhaps the northern districts of Pamplemousses and Rivière du
Rempart where, as we have recently shown, chikungunya-related excess mortality was the
highest in 2006. This would mean that much of Mauritius is still below the chikungunya
herd immunity threshold. This suggests that herd-immunity alone cannot explain the nonrecurrence of chikungunya in Mauritius since 2006. Other explanatory factors include (i)
the successful mosquito control programs carried out by Mauritius authorities, (ii)
community-based clean-up programs, and (iii) unfavourable climatic conditions.
Our study highlights the need for continued vigilance should the virus make its entry in
Mauritius again as much of the island is still below herd immunity levels. Also
seroprevalence studies are essential to confirm prevalence levels in Mauritius. Our efforts
in this direction continue.