Does the world have an alternative to herd immunity in combating Covid-19?
In this article, I tell the story of what SARS-CoV-2 does when it arrives in a population that it has never met before. SARS-CoV-2 is the name of a coronavirus that causes the disease Covid-19.
The emergence of SARS-CoV-2 in the last few months has disrupted human civilization in many ways. This article describes the different approaches to managing the impact on the population.
An unwanted visitor
Imagine a densely-populated island and a visitor, infected with Covid-19, arrives. The virus spreads and infects many people. Some come down with a serious illness, while others are hardly affected.
A person who is infected can only transmit the disease to someone else for a period of several days and only by close contact.
As more and more of the population is infected, most people recover and become immune.
A carrier of the virus is now unlikely to meet someone who is not immune from the previous infection. At this point, the virus dies out. If a carrier does not come into contact with a non-immune person during the period the carrier is infectious, then the virus will die out.
A large number of persons have been infected but a substantial part of the population still has not and is not yet immune.
The population is divided into those who have been infected and are now either dead, or alive and immune, and those who have not been infected and are therefore not immune. There are no more carriers and the virus is gone.
If someone carrying the virus arrives on the island, another epidemic would not start as too many people are immune. A few people might get sick but the virus cannot spread rapidly.
Once a population reaches this status, it has achieved herd immunity. About 60% of the population must be infected in order to achieve herd immunity.
The case for and against lockdown
Consider an immediate government lockdown. Most people stay inside and have very little human contact; children do not go to school. Most economic activity closes down. People keep their distance from each other and wash their hands regularly.
These actions greatly reduce the rate of infection. After the society is locked down for a few weeks, the virus stops spreading and disappears from the population.
Society can go back to work. I will call this limited herd immunity. But limited herd immunity is unstable and vulnerable to more waves of infection.
In limited herd immunity, perhaps only around 10% of the population has been infected -- not enough for true herd immunity at 60%. If no action is taken, the infection continues to spread until immunity reaches 60%.
If the lockdown is re-instated, then the spread of infection will again slow down or stop altogether.
However, until true herd immunity is reached, the population is always vulnerable to new waves of infection. This is the second wave that is much feared and discussed.
True immunity can also be achieved through public health measures like vaccination. Once 60% of the population has been vaccinated, the virus will not spread quickly and the number of infected and sick people can be managed by most hospital systems.
Without a vaccine, most governments have opted for lockdowns. However, in order to prevent second waves of infection, they will need strict checks on people entering the country.
Covid-19 appears to be spread by asymptomatic carriers. The only way to protect a country is to test travellers at the border before the entrance.
Otherwise, there is always the risk of becoming infected during travel and creating a second wave of infection in a country that has only achieved limited herd immunity.
Countries that have not reached true herd immunity can begin extensive testing and contact tracing.
The testing identifies someone who is infected; the tracing identifies all contacts that the infected individual has had two weeks prior. Tracing identifies and quarantines all contacts. This will limit infection in the population.
There are three possible outcomes when SARS-Cov-2 arrives in a population:
No special effort is made to reduce the spread of the virus and one reaches true herd immunity by allowing infection to spread rapidly through the population.
Sweden has adopted this strategy, coupled with a special effort to protect old and vulnerable people. Once herd immunity is reached, the old and vulnerable can safely re-enter society.
The cost of this strategy is the potential for a hospital system ill-prepared to handle the number of possible severe cases and corresponding high numbers of death.
Strict lockdown leaves the society with limited herd immunity but exposes everyone to the risk of later waves of infection. Waves of re-infection can be prevented by testing and contact tracing as well as strict border controls.
China and Germany are close to achieving this status. South Korea and Taiwan are the best examples of success. Limited herd immunity is achieved at a much lower level of infection but leaves the population still vulnerable to possible waves of the virus.
Loose lockdown will achieve limited herd immunity, but it takes longer and allows a higher number of deaths.
The percentage of infected and hence immune persons in the population will be higher than the strong lockdown but less than the first case of doing nothing. The United States is following a program of loose lockdown.
If one starts on lockdown then it is necessary to stick with it to achieve limited herd immunity. Once the number of deaths and new infections begins to decline, a population will have achieved limited herd immunity.
Once that begins, the society starts approaching herd immunity proportional to the level of lockdown implemented. But anything less than full herd immunity leaves the population exposed to future waves of infection.
There are only two ways out of this dilemma:
1. Discovery of a vaccine and vaccination of enough of the population to establish herd immunity.
2. Mutation of SARS-Cov-2 to a less infectious or a less virulent form. Then Covid-19 becomes another version of the flu. It is impossible to know how long either of these might take to emerge.
Forrest Cookson is an economist who has served as the first president of AmCham, and has been a consultant for Bangladesh Bureau of Statistics.