Fighting the Covid-19 Pathogen: An Explainer

Fighting the Covid-19 Pathogen: An Explainer

A Vadodara street during the lockdown: Epidemics cannot be permanently warded off by lockdowns | Vijay Barot (Wikimedia)

The phrase “herd immunity” has acquired such a large connotation that it has become difficult to look at it objectively.

Is herd immunity a process that can be used as a strategy? No

It is neither a process nor a mechanism. It is a description of a particular state. It is merely a descriptive concept. You cannot use such a concept as a tool to plan a strategy or to influence the course of nature. It is a state in which the spread of an infection in the community slows down from an epidemic rate to become an endemic disease due to the development of infection-specific immunity in the population.

The term ‘epidemic’ describes a phenomenon of a rising infection rate in a community. During the epidemic of an infection, each infected person spreads the disease to more than one person. In such a situation the prevalence of the disease will increase in number with time. Mathematically, if this is plotted against time it will have an exponential curve. However, this will not continue forever. It will eventually start to slow down and then achieve a state of equilibrium. It is like a chemical reaction between two reactants, the process continues until they reach a state of equilibrium. Knowledge about this reaction can potentially shift the final state of equilibrium in our favour.

What slows the spread of the infection in a community? 

As the pathogen spreads through the community, the exposed and susceptible individuals in the community develop an infection. Depending on the type of pathogen and the people’s response, a proportion of the population will die from the disease and the rest of the infected will recover with immunity. As the number of people who have developed immunity increases, the pace of the spread will start to drop. Eventually this interaction between man and virus comes to a state of equilibrium. In this state, an infected person on an average infects one or less than one person and such an infection is called an endemic infection. In this situation, the disease is still present in the community in a few individuals but it is not spreading at an exponential rate. 

This state of equilibrium in which the epidemic disease becomes endemic happens when herd immunity is achieved, a state in which a majority of the populace has developed antibodies against the virus. This is the natural course of events that will happen if we do not have knowledge regarding the mechanisms that influence the balance of this equilibrium. In this situation, neither the viruses nor the humans are eliminated. Their interaction reaches a state of equilibrium after a proportion of the populace is infected and antibodies are produced. 

What are the mechanisms that could change the course of these events and influence the final equilibrium state (herd immunity)?

Antivirals:

If there were an antiviral molecule then such a substance would treat the infection and prevent the pathogen from causing complications or death. Even in this situation, the medicine would not affect the rate of spread of the infection or the number of people who are going to be infected during an epidemic spread. It can only help us to treat the infection once it has happened. Further, this medicine will not change the time at which the equilibrium point will happen (the transition from epidemic to endemic). Thus, if we have the required knowledge to make an effective antiviral when a new virus is spreading through the population, then this medicine will change the following events in the natural course of things: 

(i) It will reduce the number of deaths at the end of the epidemic; and 

(2) It will reduce the number of hospitalizations at the end of the epidemic. 

However, there will be no significant effect on the time point at which this epidemic will end and there will be no significant effect on the number of people who get infected by the time the epidemic ends.

Vaccines:

Vaccination works by injecting a portion of the virus particles inside our body. Such particles don’t cause infection or disease but stimulate our immunity similar to a real infection. The body responds as if there is an infection and mounts antibodies against it. 

If this injection were to be administered to a naïve population, then this vaccine will have changed the following events in the natural course of things: 

(i) It will reduce the number of people who get infected by the time the epidemic ends. This is because a majority of them will already have antibodies even before the virus has been able to infect them. In this way, vaccination can shift the centre point of equilibrium in favour of the humans; 

(ii) It will affect the time point when an epidemic will end; vaccination will shorten the duration of the epidemic by making very few people available for the virus to infect; and 

(iii) There will be fewer deaths and less hospitalization.

With Covid-19, we do not have any effective antiviral that could kill the pathogen. We do not have any vaccine either that could artificially stimulate antibody or immunity production at this point in time. In short, we do not have any knowledge about mechanisms that strongly influence the natural course of events in favour of humans. If we face this virus with this knowledge base, then the events will unfold as they are.

Physical distancing, hygiene and universal masking:

Then what is the role of physical distancing, respiratory hygiene and universal masking?

Pathogens like the coronavirus that are shed in human oral or respiratory secretions can spread easily due to community living. If we were to live a solitary life in the forest with only rare contacts with other human beings, then such a pathogen will not be able to establish an epidemic among humans. The success of these respiratory viruses is due to our social life. A community with a dense population and networks of human interaction will have a high rate of transmission. In such a situation a large proportion of the community must carry immunity to break the spread of the virus. When interventions like social distancing, universal masking and respiratory hygiene are implemented, the transmissibility of the pathogen drops. 

The following changes will happen in the natural course of events: 

(i) It will shift the equilibrium in our favour and reduce the number of people who are infected by the time the epidemic ends; 

(ii) It will slow the rate of infection and could also end the epidemic by reaching a point where an infected individual infects less than one other person due to heightened respiratory hygiene among the community; 

(iii) However, these interventions must be followed as long as the pathogen is present in the human population. Once the strategies are relaxed, the equilibrium will shift towards the original point if the reaction were to happen at a ‘natural’ rate. 

Based on the complexity of their social milieu, each community should identify respiratory hygiene measures that can be sustainably followed at least until a vaccine becomes available.

Lockdowns:

What is the role of nation-wide lockdown measures?

Such measures will spread out the populace and reduce the transmissibility of the pathogen. But similar to other ‘distancing’ measures, the effect of lockdowns on the pattern of the epidemic will vanish once the measures are reversed. They can transiently and effectively arrest an epidemic, buying us valuable time to plan and implement strategies on how to safely face this phenomenon once it is allowed to unfold. These measures have no role in permanently warding off the epidemic from the community. 

It must be acknowledged that we have very little knowledge about how to eliminate the virus without significantly altering our social edifice. The community transmission of infection and its consequences are unavoidable due to this limitation. The potentially modifiable factor in this phenomenon is the ability of the health care system to take care of people when they are infected and seriously ill. Oxygen is the most important medicine in this type of disease. Most patients will just require oxygen therapy for a brief period of time as a bridge to recovery. Temporary field hospitals must be established with basic facilities to handle crisis situations. This level of preparedness is absolutely necessary to prevent a large number of avoidable deaths. 

Our fight should be proportionate to the knowledge we have about the process. We would incur more losses if we get into a fight without knowing the limitations of our strategies.

Suresh Manickavel, Lung Transplant Physician, Jackson Memorial Hospital, University of Miami, United States
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