446 YANNI XIAO, TINGTING ZHAO AND SANYI TANG
the impact of media coverage on the transmission dynamics of infectious diseases
[19, 11, 10, 24, 25]. To model the reduction in contacts due to behavioral change,
as awareness of the presence of the disease, functions of the number of infectives
are formulated to response to the reported information of cases [28]. The negative
exponential function βe
−mI
or βe
−α
1
E−α
2
I−α
3
H
, where H denotes hospitalized in-
dividuals, has been embedded into the transmission term by Cui et al. [10] and Liu
et al. [19] to examine the impact of media on the spreading of the disease. While
Cui et al. [11], Tchuenche et al. [24] and Sun et al. [23] incorporated non-linear
function of the number of infectives, c(I) = c
1
− c
2
f(I), in their transmission term
to investigate the effects of media coverage on the transmission dynamics. Liu et
al.[19] obtained the potential for multiple outbreaks and sustained oscillations of
emerging infectious diseases, and Cui et al. [10] concluded that multiple positive
equilibria are possible when the media effect is sufficiently strong.
However, these changes of individual avoidance and contact patterns, as aware-
ness of the presence of the disease, are response to not only the reported information
of cases but also the changing rate in case number. During the early stage of out-
break of an emergent infectious disease like A/H1N1 influenza and SARS [18], the
reported high growth rate of cases puts great pressure on people and cause people
to stay home or wear face masks when going outside. While as disease further
spreads, the massive cases number exhibits great influence on people’s behaviors.
So, during the whole outbreak people are aware of presence of disease either in the
case number or in their changing rate or both reported by mass media.
The purpose of this paper is to formulate a particular function to describe the re-
duction factor induced either by large number of cases (I), or by significant changes
in the number of cases (dI/dt). Awareness through media and education plays a
tremendous role in changing behaviors or contact patterns, and hence in limiting
the spread of infectious disease. Whether this media function induced by both cases
and changes in cases affect the size of an epidemic outbreak or the peak time, or
bring interesting findings remains unknown, and falls within the scope of this study.
We then examine the system with incident function dependent on both cases num-
ber and their changing rate. Firstly we convert this implicitly defined system into a
explicitly defined system, which is actually a piecewise smooth system [4, 6, 29], by
using the property of the Lambert W function [9]. The piecewise smooth system is
then theoretically and technically analyzed. Finally the epidemiological conclusions
on effect of media coverage are addressed.
2. The SIR model. We consider the dynamics of susceptibles S, infecteds I and
recovereds R. Let Λ be the (constant) recruitment rate and µ be the natural
death rate of the population. Assume that β is basic the transmission coefficient,
γ is the recovery rate from infection, and α is disease-related death rate. Such
media/psychological impact, as awareness of the presence of the disease, is simply
described by an exponential decreasing factor, resulting the transmission coefficient
as βe
−M(I,dI/dt)
, where
M(I, dI/dt) = max
0, p
1
I(t) + p
2
dI(t)
dt
, (1)
where p
1
, p
2
are non-negative parameters to measure the effect of media/psychologic-
al impact of media reported cases and changing rate. Then we have the model