Herd immunity for COVID 19. Is it a mirage? Will we ever achieve it?

How will the COVID 19 pandemic end? When will the the world achieve herd immunity against COVID 19? Is herd immunity for COVID 19 a moving mirage?

What is herd immunity for COVID 19?

Herd immunity for COVID 19 is defined as the protection from an infectious disease (COVID 19) that is bestowed on a group of people (the herd) when a majority of people in the group have adequate immunity against the offending pathogen either after having recovered from a natural infection by the pathogen or by vaccine induced immunity.

How many people need to be immunized for herd immunity?

The number of people in a group who need to be immune to achieve “herd immunity for COVID 19” depends on the infectivity of the disease causing pathogen. If the infectivity of the offending pathogen is low (it does not spread easily), then, if approximately 50% of the population has immunity it will protect the herd. If on the other hand if the pathogen is highly infectious (spreads easily like Measles), then approximately 95% of the population needs immunity for the whole herd to be protected.

The magic percentage figure for herd immunity against an infectious disease thus varies from disease to disease and  is determined primarily by the infectivity of the pathogen that is causing the disease. For herd immunity in COVID 19 disease the magic figure to achieve herd immunity is about 80%. This means that if 8 out of every 10 individuals have immunity against  the disease, then the chain of transmission of the disease will be broken and the disease is very unlikely to spread further as the 2 unprotected people in the group of 10 will be surrounded by protected people and therefore the disease will  not spread and the current COVID 19 pandemic will be stopped in its tracks.

Why is getting infected with SARS-CoV-2 to “get it over with” not a good idea?

With some other common infectious diseases, like Chickenpox and Measles, before a vaccine was made available, people sometimes intentionally exposed themselves to the disease causing virus by being in close proximity with those already infected so that they also developed the disease and then became immune as their own immune systems conquered the pathogen and bestowed immunity.

This may be an acceptable practice for less severe diseases like Chickenpox or Measles but is very dangerous for a disease like COVID 19 because its mortality is very high especially in succeptible   groups such as smokers, the elderly, those with cancer, those with Diabetes etc.

The precise mortality rate for COVID-19 is yet unknown, but currently available data suggests that it is approximately 10 times more than the mortality rate for common flu (Influenza).

Is herd immunity for COVID 18 a moving mirage?

The magic percentage figure for heard immunity is not a static target. It may be a moving mirage for the following reasons

• Like all other viruses, the COVID 19 virus also mutates. Every new variant may not be covered by the current lot of vaccines. If these new variants (which the current vaccines may not protect) spread in the community rapidly then herd immunity may never be achieved.
• There is a lot of “vaccine hesitancy” among people. Achieving an 80% protected population may take time. The longer it takes the more likely that the immunity of the “early vaccine recipients” would begin to wane.
• The vaccine may not induce an adequate immune response in immunocompromised people so 80% protection despite vaccination may not be achieved.
• If the immunity wanes with time, then 80% may never be achieved

The world does not need to hit the magic 80% figure of herd immunity before transmission of the disease begins to slow down. With less and less transmission, fewer and fewer people will get exposed to the virus, and if those who do get exposed are all  vaccinated,  fewer will become seriously ill or  die. The pandemic will slowly peter out and fade away without the world ever reaching the magic figure of 80% protection.

The fading out process will be longer without herd immunity for COVID 19 but will be significantly shorter if herd immunity is achieved.  An analogy can be drawn to putting out a camp fire with water. If a couple of buckets  of water are poured on to the fire it will be extinguished immediately . If however water is sprinkled liberally on to the camp fire, it will be burn less intensely  for a while and then gradually die down.

If herd immunity for COVID 19 is achieved rapidly, then, the spread of the pandemic will be halted quickly and its it is good for the world. If however,  herd immunity is is not achieved, even then, all hope is not lost. The world can still overcome the pandemic but it will take much longer. Till the pandemic dies down it is safe to continue to wear a mask, maintain physical distancing and follow hand hygiene practices.