Coronavirus: Community Transmission and Herd Immunity


Many countries went in lockdown mode as early as in the stages of community spread of Covid19, to keep the number of cases low. Lockdown has eased now in many parts of the world as we need to recover from the economic impact of it. But the ease of work means a higher progression leading to community transmission as we step out of our homes and meet people. Looking at the Indian numbers and traceability, its seems India is already in community transmission stage, but government just not announcing it.

I would like to explain two things here: Community transmission and community spread which are linked to different stages of disease spread. There are four stages of disease spread. Stage 1 comprises of cases which come from a foreign land and show no origins of disease in that local area. In the case of covid19, mostly passengers who flew from other countries were reported positive. Community spread is stage 2, in which these foreign returned people met others locally, hence circulating the disease. The most important thing here to note is that in this stage we can contact trace the patient or carrier who exposed this virus to locals with whom he/she came in contact. In this stage p who were tested, positive are those who had contact with a foreign returned person, like for example in a family get together, or dinner, etc. This stage is manageable and cases can be tracked, though numbers keep rising. The control measure in this stage generally includes finding people who came in contact with a person with travel history or disease positive, identifying them, isolating them, and quarantine. Community spread was the stage when nations went in complete lockdown.

Community transmission is stage 3 in which, one can not track the source of infection in a patient. During this stage person who doesn’t have travel history or met people with covid19, will start testing positive for the disease. The outbreak will escalate faster in clusters and communities, with people randomly start testing positive for the disease. Stage 4 is even more severe when the disease outspreads to the whole county with several clusters and communities of infection across parts of the country. A huge number of people start falling sick due to infection and deaths will increase. In the last two stages, managing disease becomes very difficult. Once the country has entered community transmission, stage 4 is inevitable. Many countries have crossed the community spread stage and entered community transmission, the most famous example being of Italy and China. Numbers for India also looks that it is entering community transmission (or we are not acknowledging the fact that we are already in this phase). Once community transmission happens, it will be difficult to stop the disease, and herd immunity might come in play, although it is debatable in the case of COVID.

To understand herd immunity, let us quickly look into an individual’s immunity and understand what happens when an infection (or pathogen or foreign agent) hits our body. When a foreign particle is recognized in the body, immune systems get alerted and starts a defense mechanism to get rid of a foreign particle with the help of specialized immune cells and parallelly prepares the body for adaptive immune response, to remember this foreign particle to prevent future infection. This memory is stored in the form of antibodies that our immune system makes against the foreign particle. Antibodies are very specific and will not work against novel foreign particles. So for each novel infection that body encounters, our immune system makes these sensitive antibodies so that it can fight specific pathogens in the future. This adaptive immunity can last for long periods to lifetime. For example, in the case of chickenpox, the immunity lasts for a lifetime, but for some pathogens (like hepatitis, common cold, etc), antibodies do not guarantee lifelong immunity. A vaccine is a technology that helps the body makes antibodies without getting an infection, this is generally done by exposing human to a non-functional form of the pathogen ( disease-causing micro-organism). When the immune system is exposed to a non-functional form of the pathogen, it is recognized as a foreign particle, and memory in the form of antibodies is generated against it, thus preventing any future infections from the pathogen.

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Herd immunity means that a large proportion of the population (typically around 70%) has developed immunity to a contagious disease, this can happen via vaccination, as in case of polio, where massive vaccination programs were carried by countries or it can happen when people have had an infection and after recovery they get immunity to infection, for example, if someone had chickenpox once, then they get immunity to it and chances of getting chickenpox again are low. In the case of Covid19, we don’t have a vaccine yet, so the first scenario of herd community is not possible. Community transmission will lead to a large population getting infected by coronavirus and people who will recover from it will gain immunity, thus leading to herd immunity. The way the body’s immune system works is that it makes antibodies against the infectious agent when introduced to it and stores it as a memory for fighting future infections.

Now the obvious question a lot of you might be thinking, that if people will get immune after recovery, then why did we have lockdown and we were focusing on reducing the number of patients. The reason being, Covid19 is highly contagious, even though it is not as deadly as some other disease, death has been about 5% of patients (worldometer), but people who are dying are not able to fight the virus and dying within days of infection, if we don’t take measures to stop this, 5% of the population is a huge number, and we cannot let our elderly, young kids, immunosuppressed people, etc. to die because of this. We don’t have treatment for Covid19 till now, so we can’t let disease fan out in our population. Also, this virus can mutate very fast, so there will be many different strains of this virus as it advances in people.

There are a lot of challenges with herd immunity and its effects. We need to know at what thresholds will we reach herd community, does it has to be 50% of the population getting infected or more? What is that threshold percentage? How much load does such high percentages of people infected at the same time put on the healthcare system? Will the antibodies made by recovered people, be effective in fighting the Coronavirus infection in the future, knowing that it is mutating fast? Also, how robust will be this memory of the immune system, and how long will this immunity last? There have been cases reported in japan that people got reinfection within few weeks after recovery.

Most importantly how fast will we reach herd immunity ? It might take several months to years to reach herd immunity. It cannot be reached with a day or weeks, it will take a long time till we attain immunity? Also, does it make sense to attain herd immunity by exposing the whole world population to such a contagious disease?

These uncertainties and limited understanding of Coronavirus make it scary to go towards herd community, as it might not be beneficial and might not help in this long fight against novel Coronavirus. Maybe we can’t even attain herd immunity by this way and focus on vaccine development. Thus, the best option we have to fight this virus is to reduce the spread of it. With the ease of movement and work, we must take necessary measures to keep ourselves and our loved ones safe from exposure to this microscopic infectious agent. We must continue following safe and hygienic practices and restrict meeting people (if possible) as humans are the spreader of this disease.

 Resources

https://www.jhsph.edu/covid-19/articles/achieving-herd-immunity-with-covid19.html

https://www.thehindu.com/sci-tech/health/who-chief-scientist-soumya-swaminathan-interview-data-is-key-to-control-coronavirus-pandemic/article31319156.ece

https://www.straitstimes.com/asia/east-asia/japanese-woman-reinfected-with-coronavirus-weeks-after-initial-recovery

image from https://www.pbs.org/wgbh/nova/article/herd-immunity/

https://www.nytimes.com/2020/04/13/opinion/coronavirus-immunity.html


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