Results of Round 2 of Capital’s sero survey likely by mid-aug
ESTIMATING SPREAD Officials working on survey design in order to cover sample representative of population; specifics of who and how many to be tested from Aug 1-5 will be sent to workers in few days
nNEWDELHI:
More than a hundred teams of the three-four members will fan out across the Capital in the first five days of August to collect blood from around 20,000 people, health officials in Delhi said on Tuesday, giving new details on an upcoming survey that could help determine how much closer Delhi is to achieving herd immunity to Sars-cov2, the virus that causes Covid-19.
The results of the second such serological study in the city are expected by mid-august.
The blood will be used to check for antibodies that indicate a person was once infected and has since recovered and now possibly immune. If more than 40-65% of the population has such antibodies, Delhi will have, possibly, reached the herd immunity threshold. This is when an infectious diseases peters out as vulnerable people dwindle.
The first citywide survey of this sort, covering close to 21,800 people between June 27 and July 10, showed 22.86% of those tested had the antibodies.
Like that exercise, the one next month will draw on samples that are representative of the city’s population – by age, gender and regional distribution. “For the second round of the survey, the number of samples to be collected from each ward and each district will be decided by the researchers from Maulana Azad Medical College, just like it was done by scientists from NCDC (National Centre for Disease Control) for the last round. We are yet to receive the break-up,” said a district official, asking not to be named.
To illustrate what some of the yardsticks could be, this person said around 30% of the samples collected in the first round were from those between the ages of one and 18 and all of those tested must have resided in Delhi for the last six months.
“The numbers of how many samples will be collected from each district and ward is collected based on the population in the area and the percentage of people who need to be surveyed – the highest are to be collected by North West district and the least by New Delhi district,” this person added, citing the previous survey design.
People who were covered in the previous survey will not be part of the sampling, according to the official. The collection of samples will be done by grassroots health workers, who will draw 3-5ml of blood from each member of a selected household. The teams will each consist of ASHA workers, an auxiliary nurse midwife (ANM), a phlebotomist or a lab technician, and a community volunteer. One team will be able to draw 25 to 40 samples a day.
“Our teams are ready, we are yet to get the guidelines for the survey. We can start collecting the samples as soon as we get the details. A meeting regarding the same is likely to happen by the end of the week,” said a district health official from another one of Delhi’s 11 districts, asking not to be named.
When results of the first citywide serological survey was held, chief minister Arvind Kejriwal said it represented the situation that existed around the end of June and the number of people who would have silently had the infection may now have grown.
How close, or far, Delhi is from the herd immunity threshold can have an impact on the precautionary curbs on some activities that have been in place to minimise spread of the disease. It could also determine whether some of the infrastructure preparedness –such as the number of beds reserved for Covid patients – is tweaked.
According to the Delhi government, the preparedness is likely to be maintained at current levels, even as current hospitalisations hover around their lowest in nearly two months. “As of now there is no plan to reduce the number of beds even post the sero survey. No such directions have been issued or discussion taken place. The Covid situation in Delhi is still being monitored very carefully by the chief minister and he has advised all officers not to be complacent and continue with all the Covid management activities as it is,” the Delhi government said in a statement on Tuesday.
“The repeated sero surveillance will help us understand how many people have developed antibodies against the infection and whether that is the reason for declining prevalence. Also, it would be helpful in studying how long antibodies last,” said Dr Jugal Kishore, head of the department of community medicine at Safdarjung hospital.
“When a statistical sample is selected, it is done on the basis of a presumed prevalence. In this case, it as the prevalence as per the ICMR survey. Now, if you have to properly observe something that occurs in less than 1 of 100 people, you would need a large sample size to see it. Now that we know the prevalence is about 23% -- and I expect it has gone up -- a smaller sample size will be good enough to observe. I believe 5 to 6 thousand samples will be enough for the second round of surveillance,” he added.