The first three months of the COVID-19 epidemic: Epidemiological evidence for two separate strains of SARS-CoV-2 viruses spreading and implications for prevention strategies

Knut M. Wittkowski

If a disease causes immunity after an infectious period of a few days only, like respiratory diseases, an epidemic extinguishes itself as the proportion of immune people increases. Under the SIR model, (Kermack 1991) for a reproduction number (Dietz 1993) (secondary infections by direct contact in a susceptible population) of R0=1.5-2.5 over 7 days (recovery rate: γ=1/7=.14), the noticeable part of the epidemic lasts about 90-45 days (R0/γ=β=.21-.36) in a homogeneous population of 10M. The period is shorter for smaller more homogenous and longer for larger, more heterogenous populations. For a given infectious period 1/γ (here, e.g., 7 days. SARS and COVID-19 incubation period plus 2 days (Lauer 2020) ), R0 also determines how long it will take for early cases to become visible after a single import (150-60 days), the peak prevalence of infections (5-22%), and how many people will become immune (55-90%). To allow for comparisons between models, an arbitrary proportion of symptomatic cases among those becoming infected (.05%) is used and 2% of cases are assumed to die. , infectious (red), resistant (green), case (orange) and dead (gray) people after a population of 10,000,000 susceptible people is exposed to 20 subjects infected carrying a novel virus. Assumptions: R0 = 2.2, infectious period = 7 days, (available from https://app.box.com/s/pa446z1csxcvfksgi13oohjm3bjg86ql )