Herd Immunity for COVID19- Who Do You Vaccinate?

 So you have access to several 100 millions of a new vaccine for COVID, But there are billions of people that need to be vaccinated. Who do you vaccinate? What is the quickest way to (a). Protect the most vulnerable? (b). Achieve herd immunity?

Let's deal with the logistics of achieving herd immunity first. For a given R0 (R-naught or Reproduction number), the proportion of population (say P) that needs to be immune in order to achieve herd immunity is given by: 

P= 1-1/R0.

Since the R0 for COVID19 is 3, the proportion of population that needs to be immune to achieve herd immunity is 2/3 or around 67%.

Yet, it is likely that for a large country of the size of say, India, a country of some 1.3 billion, you'd be struggling to lay your hands on nearly 900 million vaccines first up.

Or even if you did, which 2/3rds would you choose to vaccinate?

And here you are on the horns of a dilemma, almost game-theoryesque in its nature. Would you vaccinate the oldest third (and therefore epidemiologically the most vulnerable), the middle aged tertile, or the children?

Think before you answer!

Keep in mind that no matter which vaccine is used, the likelihood of elderly subjects developing immunity as a result is much lower than younger subjects. It's just the ageing immune system responds poorly to almost any antigen- be it natural or vaccine-carried- a phenomenon called immune senescence.

OTOH, children respond brilliantly to vaccines. National immunisation schedules are premised on this phenomenon. What's more, they then stop spreading the putative infective agent to the rest of the population, thus reducing its overall transmission to the older, vulnerable subjects.

To illustrate, when conjugate vaccines for Pneumococcus (PCV7, followed by PCV10 and PCV13) were introduced for childhood vaccination in 17 European countries, the occurrence of invasive Pneumococcal infections over the next 5 years (2011-15) fell in people over the age of 65- the most susceptible population-by over 75%, for the strains that were included in the vaccines, but increased for the strains that were not included. Overall disease burden among the elderly was at least moderately lower, simply due to childhood vaccination.

It follows therefore that if decision makers opt to vaccinate the older thirds of the population with the first few hundred millions of COVID vaccine, control of the pandemic would be by no means assured. In fact, failure is guaranteed, as these ageing group will respond poorly to the vaccine and will continue to be vulnerable to unfettered transmission from the young.

OTOH, vaccinating the group which is most likely to transmit the virus- the young- is likely to achieve the holy grail of herd immunity more quickly, due to the fact this younger population are more likely to respond to the vaccine and stop spreading it to the elderly.