How Contagious is COVID-19? (Transmission, Spread, and R0)
Transmission of SARS-CoV-2 occurs via droplet transmission, contact transmission, and aerosol transmission. Droplet transmission occurs when respiratory droplets produced when an infected person coughs or sneezes are inhaled by a person nearby. Contact transmission occurs when a person touches a contaminated surface and then their mouth, nose, or eyes. Aerosol transmission occurs when respiratory droplets containing the virus mix into the air and then are inhaled. COVID-19 is stable for up to 24 hours on cardboard, 2-3 days on plastic and stainless steel and up to three hours in aerosols, which include fog, mist, dust, air pollutants, and smoke. Therefore, it is possible to get infected by touching contaminated objects or through the air.
The incubation period is the time between infection and symptom onset for an illness. Estimates for COVID-19’s incubation period vary from 2-14 days, but it is generally assumed to be around 5 days. There is more debate about the latent period, which is the time between infection and infectiousness. It is now thought that people can be infectious before showing symptoms, and so the latent period is shorter than the incubation period.
An imported case occurs when a traveler is infected in one area and is reported as sick in another area. Local transmission occurs if that traveler infects others, or if there is a cluster of cases locally and the spread is easily traced. Community transmission occurs when there is no clear source of infection.
Infectivity can be measured using R0. R0 is important epidemiology jargon, short for reproduction number. It is the number of cases, on average, that an infected person will cause during their infectious period. So if R0 =2, then an infected person will infect an average of 2 other people while they are infectious. There are two important variants of the R0. The basic reproduction number represents the maximum potential of a pathogen to infect people – basically what would happen if an infectious person entered a community with no prior immunity. The effective reproductive number describes the current vulnerability of a population based on whether people have immunity thanks to vaccination or prior exposure. The effective R0 decreases over the course of the outbreak. Note that both basic and effective reproduction number depend on factors such as environment and demographics in addition to the pathogen’s infectiousness. The goal of public health interventions is to bring R0 down to less than 1, as this would cause the disease to die out over time.
The seasonal flu has an R0 ranging from 0.9 – 2.1. There is a lot of debate about the R0 of COVID-19, with estimates from more recent data ranging from 2.7-4.2. The variance in these estimates is largely due to differing model assumptions and a lack of data. For example, models which assume the possibility of being infectious before symptom onset have estimates that are around 0.5 higher.
These high R0 estimates mean there is much greater potential for spread of COVID-19 than for the flu. How much greater? For the purpose of this example, let’s say that the flu has an R0 of 1.5 and COVID-19 has an R0 of 3. After three cycles of infection, 11 people have had the flu, and 40 people have been infected with COVID-19. After ten cycles of infection, this becomes 171 people with the flu, and over 88,000 people with COVID-19.
3D Models from:
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