Basic reproduction number, or R0 or contagiousness
Dr K K Aggarwal
President CMAAO, HCFI and Past National President IMA
R0 represents the average number of susceptible individuals an infected person will transmit the disease to.
R0 = Attack rate X Contacts
Attack rate (the percent chance a given contact will get the disease).
If the R0 is < 1, a disease outbreak should wane over time, and if it's > 1, cases should continue to increase.
Seasonal flu has an R0 of around 1.5. The Spanish influenza of 1918-1919 had an R0 as high as 2. Chickenpox, which is fairly infectious, has an R0 of around 5.
To predict how terrible a new disease will be, you really want to look at both the basic reproduction number and the case fatality rate.
Historical reports put the case fatality rate of Spanish flu as high as 10%.
HIV before treatment was available, with an R0 of around 6 globally and a near 100% mortality rate. Smallpox, with an R0 of 5 and a mortality rate of 30% in the unvaccinated. Bubonic plague: R0 of 3, untreated mortality rate of 60%.
Has R0 of 2.5 and a reported case fatality rate of around 2%.
Case fatality rate is defined as the number of fatal cases divided by the number of total cases.
In COVIOD 19 we probably captured fatal cases accurately; people who are that sick generally end up in hospitals. But we may be missing the number of total cases by huge margins, perhaps even an order of magnitude, because asymptomatic and mildly symptomatic people may not be getting tested. If this is the case, we should see the case fatality rate decrease as screening improves.
How to change the R0 number
One can do that by addressing the two elements inside it: the number of contacts an infected person has and the attack rate of the disease.
Limiting potential contacts can be achieved through isolation and quarantine.
Attack rate can be reduced by wearing masks, handwashing, and, of course, vaccination, were a vaccine to become available.
This depend on identifying cases, and it is still an open question as to whether transmission can occur in the asymptomatic period. The latent period is 5 days.