How the Covid-19 Epidemic Spreads and How It Will Die

Epidemics result when each person infected by a disease infects more than one other person. Then the number of persons infected grows day by day. But if those who are infected survive to become immune to the disease, then, as disease spreads through the population, the number of persons that each infected person infects must fall, ultimately falling below one. At that point the population is said to have achieved herd immunity. From there on, the number of persons infected daily declines until the spread of disease is entirely extinguished.

Because infectious diseases have an incubation period during which an infected person remains in apparent good health, observable or reported infections follow actual infections by the duration of the incubation period. In the case of Covid-19, the incubation period is usually between one and two weeks. Thus the occurrence of Covid-19 herd immunity precedes the peak of observable infections by one or two weeks and the peak in reported deaths by as much as a month.

Many communities in North America and elsewhere, are reporting declining daily totals of new  Covid-19 infections. That means that herd immunity has been achieved and the epidemic is dying. In New York City, herd immunity is reported to have been achieved with just 21% of the population having been infected.

The spread of the disease depends, however, on social dynamics and other measures to control disease spread. Currently, social dynamics in most communities have been radically altered by so-called social distancing in public places, lock-downs, and quarantines. The use of face masks and the use of disinfectant both indoors and out have also affected disease spread. Once such measures are relaxed or eliminated, disease spread among those still susceptible will occur more readily and the number of persons infected by each newly infected person will rise.

Relaxing or ending measures to control social dynamics thus raises the number of persons that each newly infected person infects to a number greater than one. As a result herd immunity is temporarily lost and the rate at which new infections occur will increase. However, the continued rise in the number of those infected will increase the proportion of the population with immunity, so that herd immunity will be regained, but with a higher proportion of the population infected.

The rise in the proportion of the population infected that occurs during transition from a period of controlled social interaction to a new period of normality is unavoidable. Adverse consequences of the transition to normality from the current period of controlled social interaction can be limited in two important ways.

First, by a progressive transition that prevents a spike in new infections that overloads hospital capacity to treat those who suffer serious adverse reaction to the disease.

Second, measures to to protect those most likely to suffer severe illness or death as a result of infection. Mostly, that means protecting the elderly, who account for the vast majority of those killed by Covid-19.

Also in need of special protection are those with conditions such as a respiratory disease, obesity or diabetes who may suffer particularly adverse reactions to the disease and should, therefore, be held in quarantine for their own protection until the epidemic ends.

Currently, the death rate due to Covid-19 in North America is probably close to 0.3% of those infected. Of those who have died, almost half in the United States, Canada, Italy and probably most other countries are elderly people suffering multiple other diseases and resident in a care home.

The future death toll due to Covid-19 could therefore be greatly reduced by better protecting the elderly, particularly those in care homes, from infection. The most important measure to achieve this would be repeated testing of care home workers and residents for Covid-19 infection to insure infected individuals are promptly quarantined.

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Understanding the Reproduction Number, Lock-downs and Herd Immunity

During a disease epidemic, lock-downs cut the reproduction number -- that is the number of people infected by each infected person -- by reducing opportunities for viral spread. This can be seen in the Canadian data, which show that the number of newly reported cases turned down within two week of the onset of a national lock-down (see figure 2). As the incubation period for the disease is approximately two weeks, that indicates that the lock-down had a more or less immediate effect in reducing the spread of the virus.

Figure 2. New reported COVID-19 cases (n=5,590

Footnote1) in Canada by episode dateFootnote2 as of April 1, 2020, 11 am EDT

When R0 is less than one, the epidemic dies down and is ultimately extinguished. Problem is, if R0 is driven down by means of a lock-down, any remaining cases or new cases from whatever source are likely to re-ignite the epidemic when the lock-down ends. That is the danger China now faces. China is dealing with this risk by testing people entering the country so that imported cases can be quarantined, and so that contacts of new cases of any source, can be tracked down and quarantined.

In time, if the proportion of the population that has been infected rises high enough, R0 will fall below 1 without a lock-down because most people are immune and cannot therefore be (re-)infected. That is why, in Britain, epidemiologists are now conducting surveys to determine how many people have already acquired immunity. If the proportion is high enough, probably over 60%, herd immunity will have been achieved. That means that even without control over human interactions, R0 will be less than 1 and the virus will be unable to spread. At that point, any new outbreak will die out spontaneously and the national lock-down can be ended.

A further consequence of making a reliable estimate of the population-wide infection rate is that it will provide a basis for an accurate estimate of the mortality rate for the disease. At present, mortality rate estimates are hopelessly skewed because the infection rate includes only those with disease symptoms who are known to medical authorities. Those who suffer the illness without reporting to a doctor, or who are infected without experiencing disease symptoms, are currently excluded from mortality rate estimates, thus elevating the apparent death rate, probably many times in excess of the actual rate.

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How China Ended the Corona Virus Epidemic -- If It Has Ended

Infectious diseases cause epidemics when infected individuals infect, on average, more than one other person, causing the total number of infections to grow despite recovery or death of those previously infected. The number of individuals infected by each infected person is call the Reproduction number (R0).

Epidemics end when R0 falls below one. The COVID19 epidemic in China went exponential in January, with an R0 peaking at almost 4.0, and then tailed off throughout late January and February.

Why did it tail off?

Simple: quarantines and sequestrations.

Quarantine means restricting people's movement. This may mean restricting or prohibiting travel from one city to another, often referred to as a "lock-down,"or it may mean closing schools, universities, and places of business, thereby keeping people largely confined to their homes, rather than crowding together in the streets, on trains and buses, and in all places of assembly where those who are infected but asymptomatic can pass on the infection.

Quarantines may extend to ordering people to stay home, or even physically barring them from leaving their homes.

Sequestration means keeping the infected out of contact with the uninfected, including their own family or others with whom they share accommodation. In Wuhan, that was achieved by the creation of many thousands of beds in isolation facilities. These facilities are not primarily treatment facilities, they are simply places where sick people are put together out of contact with healthy members of their family and community.

The effectiveness of sequestration can be greatly enhanced if infected and infectious, but asymptomatic, individuals can be identified. Simple and rapid COVID19 tests have been developed, although control of the epidemic in China seems to have been achieved prior to widespread use of testing.

China's success in controlling COVID19 is proof of the effectiveness of China's authoritarian form of government in handling a crisis of the kind that would be experienced in the event of a biological WMD attack. As the virus spreads around the world, we will learn whether Western governments are can achieve by gentler means, China's remarkable success in containing this novel Corona virus.

But China's success remains to be confirmed. As reported here, some in China say the victory over the bat flu is FAKE.

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