Serological surveys

What are serological surveys

Context: The Indian Council of Medical Research conducted two serosurveys: May 11 to June 4 and August 17 to September 22.

What is serological survey?

• Serological surveys are used to detect the prevalence of antibodies against COVID-19/any diseases.
• Their purpose is to measure the proportion of a population already infected as evidenced by prevalence of antibody against the disease.

How serological surveys are carried out?

• Statisticians stratify the population and select a random sample from all strata so that the prevalence figure obtained is representative of the whole population.
• Random samples are tested from the entire population then, the data are extrapolated to the whole population.

Why antibody is tested?

• Antibodies are the evidence of the host’s response to virus infection.
• Their presence in the blood-serum confirms past infection.

Why the data derived from serosurveys must be interpreted with caution?

• Issue with Asymptomatic cases: Asymptomatic cases constitute more than 80% of those infected with the virus. In people with asymptomatic infections, these antibody levels decline over time and become undetectable by 60 days after proven infection.
• Swift disappearance of S2 antibodies: In a study on exposure-prone healthcare workers in Tennessee, nearly half the subjects with S2 antibodies became negative in two months.
• Inaccuracy: The latent period between infection and the appearance of a detectable antibody is about four weeks. Those who got infected recently before the survey may not be covered.
• If taken at face value, serosurveys may not reflect the true level of antibodies prevalence.

What is the way forward?

• The predicted herd immunity level needed to end the epidemic was 60%. In India, according to the serological surveys, half of the herd immunity level required to end the epidemic was already reached by mid-September.
• However, during the festival the Governments must enforce strict norms regarding crowding, especially inside buildings.
• This will ensure safety of the individuals, family members particularly senior citizens.