50% of Delhi population has Covid antibodies: What the latest sero survey shows

For an infection like Covid-19, it is thought that herd immunity would be achieved when there is a seroprevalence between 60 and 80%

At least half of Delhi’s population may have been exposed to the Sars-Cov-2 virus, the latest serological study in Delhi suggests. Here is what we know about the survey:

Representational Image. (HT file)

• 56% of the 28,000 people sampled during a citywide survey tested positive for antibodies.

• The number was particularly high in the south-east district at 62%, and now appears to fall in a territory that experts believe is close to the herd immunity threshold.

• Herd immunity is when enough people have immunity – estimated to be 60-80% of the population in the case of Sars-Cov-2 – to a disease that leaves very few vulnerable people for it to spread to, slowing an outbreak.

• The figures are as per a preliminary report of the fifth round of serological surveillance that was submitted to the Delhi health secretary on Monday.

• The detailed report is likely to be submitted by researchers conducting the study from Maulana Azad Medical College by January 28.

• The survey was conducted between January 11 and 21.

• Experts said the survey’s results are in sync with infection trends seen in the capital

• The city went through its worst phase of the outbreak late October and early November, and the number of new infections has now plummeted to a level not seen since when Delhi was still in some form of a lockdown in the summer.

• A seroprevalence of 56% would mean a good number of people have been exposed to the infection.

• For an infection like Covid-19, it is thought that herd immunity would be achieved when there is a seroprevalence between 60 and 80%.

• A mathematical model by the universities of Nottingham and Stockholm has shown that the number could be in the low 40s in the context of restrictions in place to slow the spread of Covid-19.

• Studies have shown that even in those without detectable amounts of antibodies, there is still some protection from the infection.

• This means the spread of the disease will be very slow and there is unlikely to be a surge in cases in the near future especially with the vaccination drive now adding to the number of people protected against the disease.

• The latest figure indicates an over 30 percentage point jump over the last sero survey that was conducted in October, before the city entered the third wave of infections.

• Serological studies can be tricky to carry out and read, with sampling design and quality of kits holding a large influence on the outcomes.

• Scientists have also cautioned that herd immunity as a concept has not been studied in the context of natural infection, and only refers to protection brought on by vaccines.

• The concept of herd immunity does not hold when you talk about natural infection.

• Herd immunity is calculated as 1-1/R0 only in the case of vaccine-induced immunity. In case of a natural infection, anyone who is infected also passes it on to others, which is not the case with vaccinations.