parameter beta = 0.2
parameter gammaA = 0.2
parameter alpha = 0.4 # fraction of asymptomatic infectives
parameter gammaE = 0.04
parameter gammaIa = 0.1
parameter gammaIs = 0.1
parameter population = 1000

compartment S = population-1
compartment E = 0
compartment A = 1
compartment Ia = 0
compartment Is = 0
compartment R = 0

S+A ->E+A  @beta
S+Ia->E+Ia @beta
S+Is->E+Is @beta

   E->A    @gammaE
   A->Ia   @gammaA*alpha
   A->Is   @gammaA*(1-alpha)
  Ia->R    @gammaIa
  Is->R    @gammaIs