Reducing the DOTS for Infectious Diseases

To measure of how fast an infectious disease is spreading, you need to know the Reproduction Number. This is the average number of people someone with the disease will infect during the course of their illness. 

• If the reproduction rate is bigger than 1 the disease will spread, and the larger it is, the faster it will spread
• If the reproduction rate is less than 1, the disease will die out, and the smaller it is, the quicker it will disappear.

The value of the reproduction rate is determined by four quantities, the DOTS:

Duration. The number of days someone with the disease is infectious

Opportunity. The average number of close contacts a typical person has each day.

Transmission, The proportion of contacts that lead to the infection being passed on.

Susceptibility. The proportion of the population without immunity.

Reducing any of these four numbers will slow the spread of the pathogen. If the reproduction rate stays below 1,  the disease will eventually go away.

How can we reduce the DOTS for the novel coronavirus?

• We can’t change the duration of SARS-CoV-2 because that is an intrinsic property of the disease when it is allowed to run its course without intervention, and we have no proven treatment at the moment.
• At the beginning of the outbreak, the susceptibility was 100% because no one had had covid-19 before, and we can’t actively change it until we get a safe and effective vaccine. However, it is expected that people who have recovered from the disease will have antibodies produced by their immune systems, which should afford them some protection against reinfection; but at what level and for how long is still unknown.

That leaves opportunity and transmission.

• We can make a big reduction on the opportunity by keeping people at home and quarantining those that show symptoms or have underlying health problems. The experience in Wuhan strikingly showed this: before the outbreak the average daily number of contacts was 15. This number dropped to 2 during the strict lockdown and the reproduction rate fell to 0.6. Consequently the disease died out.
• Social distancing also has a big effect on reducing the transmission rate. Washing hands and surfaces, covering mouth and nose with a tissue when coughing and sneezing, and probably wearing masks also help to minimise it.

Other issues that need to be taken into account are: silent spreaders (those with no symptoms who are nevertheless passing on the infection in all innocence); super-spreaders (those with a high viral load who, because of their work or lifestyle, have very many contacts); the different immune responses of men and women, young and old; genetic mutations of the virus, which have already been identified and used to show the convoluted journeys different strains have made around the world.

Readers seeking more of the details behind this post might be interested in the timely book The Rules of Contagion by Adam Kucharski, published on February 13th. He must either have had inside information or a good fortune-teller.