Updated: March 22, 2021 8:51:44 am
The COVID vaccine in India was authorised for “emergency use”. The vaccine must be used efficiently for controlling the pandemic. Any vaccine has two purposes: One, to protect the individual against the disease, and two, to produce “herd immunity” to stop its transmission. Herd immunity is possible only in diseases which are transmitted from one human to another, for example, measles, diphtheria and COVID. In such diseases, herd immunity is very important in stopping the transmission. The immunity level needed to stop the transmission depends on how infectious the disease is. For example, in diseases like measles, herd immunity is achieved at around 90 per cent as each measles case can transmit the disease to 10-15 people. In case of COVID this is at about 60-70 per cent as one case of infection transmits the disease to about two to three people. Many countries are planning to vaccinate 70 per cent of population to achieve herd immunity.
The second concept to understand is that herd immunity is a local concept, not a national concept. For example, if city A has 80 per cent vaccine coverage and city B has only 10 per cent vaccine coverage then disease transmission in city A will stop but not in city B even if the country’s average immunisation is 70 per cent.
Immunisation is needed where the disease transmission is high. We give Japanese Encephalitis (JE) vaccine in eastern Uttar Pradesh and Bihar where the risk of the transmission is high, not in rest of India as the risk of transmission is low there. The same principle has to be followed in the COVID vaccination drive. We must first vaccinate the population in districts which have a high COVID load so as to cut down the transmission in those districts.
Take, for example, a hypothetical country where there are 100 districts of one million population each. In this country, the COVID disease distribution is such that five districts have 60 per cent of the total COVID cases, another 10 districts have 20 per cent of the cases, and the remaining 85 districts have 20 per cent of the case count. Now, you have a limited number of vaccine doses available, say 30 million. In such a situation there could be two ways to distribute this vaccine quantity. Model A where each of the 100 districts gets the vaccine to cover 15 per cent of its population with two doses. This is a fair and equitable distribution. Model B where we give all the 30 million vaccine doses to cover fully the 15 districts (two doses for the 15 million people) which are producing 80 per cent of the COVID cases. This implies no vaccination in the remaining 85 districts which accounted for 20 per cent of the cases. This model is highly inequitable and many political objections are likely to be raised. From an epidemiological point of view, however, Model B is much more superior than Model A as it will rapidly achieve herd immunity in the 15 districts which will reduce the cases by 80 per cent. In the next phase, when more vaccines are available the rest of the country (85 districts) can be covered in phased manner. This is nothing but what is called as “80/20 rule” in management or the Pareto Principle, which states, in general, that 20 per cent of causes or units lead to 80 per cent of the problems in most systems.
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If you use Model A, where 15 per cent of the population of each of the 100 districts is vaccinated, there will not be much herd immunity anywhere in the country. And the disease may not decline at all or very gradually. So equity in vaccination is not an efficient or desirable solution in “emergency” situations for which the current vaccines have been approved. In this emergency situation the objective of the public health action should not be the protection of each individual but to rapidly reduce the transmission of the disease.
Our analysis shows that of India’s 739 total districts, the top 50 districts (7 per cent of the total districts) have about 51 per cent of total cases, 68 per cent of active cases and 55 per cent of deaths. These 50 districts have large cities like Delhi, Mumbai, Chennai, Ahmedabad etc. Hence drawing on lessons from the above hypothetical example India must focus the available vaccines to cover 70-80 per cent of the population in these top 50 districts. This will build herd immunity and cut down the transmission very rapidly in those districts. It will end the emergency situation in the country. Secondly, in these 50 districts people have seen a lot of cases and death due to COVID and therefore will be willing to take the vaccine. On the other hand, in the rest of the districts where COVID cases are less, people may not be very willing to take the vaccine.
Thus there is strong epidemiological justification for focusing on the top 50 districts, and vaccinating all people or at least 70 per cent of the people in those districts starting with the older people, those with co-morbidities and then the general population.
Vaccinating healthcare workers and COVID warriors is a good strategy initially as it builds confidence. But we must focus on the top 50 districts. This will yield rapid results in controlling the COVID epidemic. The government and scientists advising the government must consider this epidemiological strategy of focused vaccination. Spreading the vaccine too thinly over the country may be a wastage of resources and may not decrease the disease spread rapidly. Such a uniform vaccination strategy cannot also justify the term “Emergency Use Authorisation”.
This column first appeared in the print edition on March 22, 2021 under the title ‘Don’t spread vaccine too thin’. The writer is the Director, Institute of Public Health, Gandhinagar, affiliated to PHFI. Views are personal
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