The second serosurvey in Delhi shows that herd immunity is not a solution and we need to continue with the public safety measures.
Delhi became the first state to complete and report a second seroprevalence survey to assess the presence of antibodies against SARS-CoV-2 in the general population.
The survey was conducted between 1 and 7 August. In the survey, 29.1 per cent of the total 15,000 samples tested, were found positive for the SARS-CoV-2 IgG antibody.
This is an increase of 6.25 per cent over the previous survey conducted from 27 June to 10 July. The midpoint of these two surveys was nearly one month apart (4 July and 4 August) and considering it takes around 1-2 weeks after the infection to develop detectable antibody, these surveys reflect the proportion of the population infected around 25 June and 25 July, respectively. Sero surveys are a scientific approach to assess the prevalence of antibodies and the spread of infection.
However, there are stated limitations of these surveys. While there is a need for caution in interpreting a standalone survey in any setting, any repeat survey in the same setting has a distinct advantage. It can help in assessing the trend in infection and effect of interventions and can help to guide the response.
There are wide variations in change in antibody prevalence between the two surveys. If we look at the trend between two surveys (see graphic), 6.25 per cent of the total population, or around 8% of the susceptible population become infected between the two surveys.
If we simply look at the numbers, 22.85 per cent of seroprevalence would mean around 45 lakh people in Delhi (the actual number would be smaller in my estimates).
What we can be surer about is that in one month between two surveys, around 12.5 lakh new people may have got infected. This is far higher than nearly 56,000 new people reported positive in this period. This implies for every person testing Covid-19 positive in Delhi, there could be 22 unidentified cases and nearly all of them asymptomatic. This ratio has also been noted in serosurveys in other cities.
It also means that the proportion of asymptomatic infection could be more than 80 per cent or even 90 per cent of the total infected. An important caveat is that these surveys measure simply the presence or absence of antibody and not level of antibodies.
The detection of antibody does not mean immunity; therefore, we do not know what proportion are protected, the information needed to arrive on herd immunity in the population.
Even if the T-cell dependent response is factored in, the most probable scenario is that proportion of population immune or protected would be less than those with antibodies. Understandably, with nearly 71 per cent population susceptible, we need to continue to follow the public health measures.
As the susceptible population is going down and those with immunity increasing, chances of new infections and progress towards herd immunity will be slower. Therefore, to reach a threshold of herd immunity (THI) at whatever level, it will take a few months more than we thought.
At the current pace, the THI levels of 50 per cent, 60 per cent or 70 per cent can only be reached by the end of December 2020, March 2021 or July 2021, respectively. With the level of personal protection and public health measures expected to be enhanced, it may take even longer.
The concept of herd immunity is more suitable for large and well-controlled geographical area and population. It is not very useful for large cities with wide travel and migrant inflow and outflow. A person living in Delhi may be protected by herd immunity. However, they would be at risk if they travel to another city. Therefore, it will be long before THI is achieved at the national level.
Even with an increase in cases at the national level, the pandemic seems to be slowing down in a few cities. It will take additional time to reach THI in those areas. We are several unknown factors about THI like the protective level and duration of antibodies as well as the effectiveness of T Cells.
All of this makes means that we should continue to follow public health measures such as social distancing, face masks and hand washing.
It has been hoped that pandemic will be controlled when 60-70 per cent of the population develops immunity—either after natural infection or vaccination. Even with THI being achieved in limited areas, we will still need safe and effective vaccines to tackle the spread of the pandemic.
A vaccine is our true hope. Till then, serosurveys should be continued to guide the response and all of us need to continue to use masks, wash hands, and keep following the approach of test, trace, treat and isolate.
Dr Chandrakant Lahariya
Epidemiologist & public health specialist