The Susceptible, Infected, and Recovered (SIR) Model for Covid-19

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Why should you attend?

While the NPIs have obviously failed to contain Covid-19 due primarily to noncompliance among the general public (which is linked in turn to general lack of understanding of the underlying science), they did end the 2019-2020 flu season about a month early and will probably result in no substantial 2020-2021 flu season whatsoever. This is because there is a vaccine for seasonal flu, many if not most people avail themselves of its protection every year, and the 2019-2020 seasonal flu could not survive the combined effects of the NPIs and the vaccine. The NPIs were however unable to end Covid-19, for which there is no vaccine, by themselves.

The SIR model can incorporate the effect of a vaccine to illustrate its effects on a contagious disease. Employers and others can use this information to encourage workers and other stakeholders to get the annual flu vaccine, and also to underscore the importance of social distancing, face masks, hygiene, and the other NPIs that have been recommended to stop Covid-19.

Areas Covered in the Session:

1.The physical and economic dangers associated with Covid-19 are obvious.

· The disease has a relatively high (on the order of 2%, lower for younger people and higher for older ones) lethality, is extremely contagious, and is often asymptomatic. People who have it might not know they can give it to others, and even survivors can suffer lifelong damage.

·Many small companies have already been put out of business because of government-mandated shutdowns whose purpose was to stop the disease from running out of control.

2.The good news is however that non-pharmaceutical interventions (NPIs) can reduce the disease's menace to allow businesses to reopen safely. They also work against seasonal flu and have already, in combination with the annual vaccine for that disease, ended the 2019-2020 flu season about a month early. We can predict reasonably that there will be no 2020-2021 flu season for the same reason, and the takeaway is that the NPIs are effective.

3.NPIs impede both Covid-19 and seasonal flu by suppressing their basic reproduction numbers (NPIs).

·R0 is the average number of people to whom an infected person will transmit the disease; if NPIs reduce this to less than 1, the disease will die out.

· Vaccination (like herd immunity) puts a substantial fraction of the population into the Recovered (and immune) category, but without the risks associated with getting the disease itself. If a portion of the population is vaccinated against seasonal flu, R0 does not need to be reduced to less than 1 to have the same effect.

4.There is not yet a vaccine against Covid-19 so NPIs are currently our only way to suppress it. These include, in descending order of preference,

·Elimination of the risk through work-from-home, distance education, distance conferencing, and even virtual tourism. Covid-19 cannot, unlike a computer virus, travel over the Internet or a telephone line. These remote work and conferencing options also offer opportunities for enormous business cost reductions.

·Reduction of the risk by, for example, curbside pickup of groceries and drive-up rather than in-person banking.

·Engineering controls such as partitions and ventilation that do not rely on vigilance and compliance for protection. Partitions between restaurant tables, for example, add distance between diners without the need for more actual floor space.  Ventilation reduces the concentration of contagious aerosols.

·Administrative controls like social distancing rely on vigilance and compliance for effectiveness. Staggered shifts reduce the number of people present, and therefore opportunities for contagion, at any given time.

·Personal protective equipment (PPE) is a last line of defense. Face masks impede the disease's spread and, if used properly, offer some protection to the wearers as well as to others but they are not a substitute for the superior controls discussed above. The risk increases with (1) intensity of exposure, e.g. by being close to an infected person, and (2) duration of exposure. As an example, it could be less risky to be 6 feet from an infected person without a face mask than to be 1 foot from him or her with one, as contagion falls off rapidly with increasing distance.

5.PPE must be selected, worn, and maintained properly to be effective.

·Respirators offer the highest level of protection, and those with exhaust valves are apparently not used by health care workers (because they do not protect patients and others). Placement of an ordinary mask over the exhaust valve can address this issue.

·Face masks impede contagion but allow a considerable amount of air to circulate around their sides. Superior masks that tie behind the head instead of relying on ear loops, such as the Durand mask of 1918, reduce this problem as do face mask tighteners that pull the ear loops behind the head. FixTheMask has developed face mask braces that provide excellent seals between the mask and the respiratory system. All these measures can make face masks more effective.

Disclaimer; no portion of this presentation constitutes formal engineering or occupational health and safety advice. It includes references to authoritative sources like OSHA and ASHRAE whose guidance I recommend as the best available.

Attendees will receive a handout of the slides and accompanying notes, along with an Excel spreadsheet on which they can use the SIR model to simulate the effects of basic reproduction numbers as well as the effects of the seasonal flu vaccine on the progress of a contagious disease.

Who will benefit:  

All people in decision-making positions with regard to

(1) Business strategy

(2) Addressing the COVID-19 outbreak, as well as those who wish to educate stakeholders on the importance of compliance with social distancing and other recommended countermeasures against Covid-19 along with the value of the annual flu vaccine.

Webinar Events
Speaker: William A. Levinson,

William A. Levinson, P.E., is the principal of Levinson Productivity Systems, P.C. He is an ASQ Fellow, Certified Quality Engineer, Quality Auditor, Quality Manager, Reliability Engineer, and Six Sigma Black Belt. He is also the author of numerous books on quality, productivity, and management.

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