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I have a simple SIS model with a non-constant population size due to disease-related mortality:

\begin{aligned} \frac{dS}{dt} &= a I - b S I\\\\ \frac{dI}{dt} &= b S I - a I - c I \end{aligned}

where $S$ and $I$ are the susceptible and infected classes, $a$ is the rate of transition back to the susceptible class after being infected, $b$ is the per-capita transmission rate, and $c$ is the disease-related mortality for infecteds.

Finding the equilibria of this model is stumping me for some reason. First, I factor the second equation to get:

\begin{equation} I ( bS - a - c ) = 0 \end{equation}

so $\hat{I} = 0$ or $\hat{S} = \frac{a+c}{b}$ must be accord with $\dot{I} = 0$. The first means that:

\begin{equation} \dot{S} = a(0) - b S (0) = 0 \end{equation}

and so $\hat{S}$ must be its initial condition, $S_0$.

However, there is another equilibrium where the disease spreads and dies out, meaning $\hat{S} < S_0$, which I assumed is equal to $\hat{S} = \frac{a+c}{b}$.

However, using simultation in $R$, this equilibrium isn't correct - $\frac{a+c}{b}$ is still off by a fair amount.

Can anyone see where I'm going wrong? Thanks!

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For $I=0$ you can have any value of $S$ to get an equilibrium position, the equilibrium equations do not impose further conditions on $S$. If you add the stability at $I=0$, then you have to add the condition $bS−a−c<0$. Outside that region the values for $I$ will increase.


If you divide both equations, you find that $$ \frac{dS}{dI}=\frac{a-bS}{bS-a-c} $$ is a separable equation, so that $$ I+S-\frac{c}{b}\ln|bS-a| $$ is a conserved quantity. That is, for the limit with $\hat I=0$ you get the equation $$ \hat S-\frac{c}{b}\ln|b\hat S-a|=I_0+S_0-\frac{c}{b}\ln|bS_0-a| \\~\\ e^{b\hat S-a}(b\hat S-a)=e^{b(I_0+S_0)-a}(bS_0-a) \\~\\ \hat S=\frac1b\left(a+W_0(e^{b(I_0+S_0)-a}(bS_0-a))\right) $$ for the limit value $\hat S$. The sign of $bS-a$ has to stay constant, $W_0$ is the Lambert-W function.

This only works for the simplest of models, more complex models will usually not give rise to conserved quantities/constants of motion.

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  • $\begingroup$ Okay, thanks. I see that at for $\hat{I}=0$ and $\hat{S}$ equal to any value, stability depends on the ratio $\frac{a+c}{b \hat{S}} > 1$, which means infections will increase before dying out. But how do I find the equilibrium value of $\hat{S}$? My simulations show that when that ratio is greater than 1, $S$ settles down to an equilibrium, but I am struggling to compute what that is. Thanks. $\endgroup$ – user_15 Nov 3 '20 at 11:10
  • $\begingroup$ Thanks for your edited response. However, I'm still not understanding how we can derive the equilibrium value of $\hat{S}$ from your answer. Is it possible? Or can we only derive the conditions for when disease will grow in the population? $\endgroup$ – user_15 Nov 3 '20 at 13:42
  • $\begingroup$ What you now have is an implicit equation. You can solve it numerically or via the Lambert-W function, which is only slightly better than giving the numerical method a name. But the value of $\hat S$ is uniquely given by that equation. $\endgroup$ – Lutz Lehmann Nov 3 '20 at 13:51

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