A new citywide blood sampling will be carried out in the first five days of every month beginning in August, Delhi’s health minister Satyendar Jain said on Wednesday, announcing a regular survey to determine how much further Covid-19 may have spread in the Capital – a strategy that could show how close the city is to achieving herd immunity and whether it needs to tweak its response measures.
The announcement came a day after the National Centre for Disease Control (NCDC) released results from the first such surveillance – known as a serological study – showing that 22.86% of 21,387 people who were tested showed signs of being exposed to the virus.
“Sero-survey identifies people who have recovered from the disease. The 23% result of the previous survey is most likely to be people who got infected around mid-June, because antibodies take around 15 days to develop. It has been more than a month now. The government plans to make it a monthly exercise. We shall be doing the next sero-survey between August 1 and 5,” Jain said.
The figures from the NCDC report, if extrapolated, could mean that over 4.5 million people — or nearly 1 in every 4 – have had the disease and recovered till June end, manifold more than the 126,323 confirmed till Wednesday. They also suggest that the disease kills only 0.07% of people infected, as opposed to the 2.94% fatality rate in the city.
The findings led experts to conclude that the peak of infections in the Capital was definitely over. “The biggest silver lining is that if we have such high population that was exposed in the past but did not largely display serious symptoms or succumbed — then it is as good as a vaccine,” Dr Sujeet K Singh, director, NCDC, told HT on Tuesday.
But, experts maintain that there is no room for complacency and people need to continue with precautionary measures such as social distancing and wearing masks.
Epidemiologists believe an infection prevalence rate of 40-65% in a particular population could be adequate to reach a level known as herd immunity, when the number of people who have never had the disease (and are not protected by antibodies that infected people build) dwindle to such an extent that an infectious disease dies out due to a lack of vulnerable hosts.
Jain, when asked about herd immunity by reporters he was interacting with, said even experts differ on this concept. “Usually, it is perceived as a stage when around 40% to 70% people have recovered from the disease. We cannot comment on herd immunity at this stage. We will get a better picture of that after the numbers of fresh cases stop emerging,” he said.
The minister added that “there definitely was spread in the community”. “Whether one can call it community spread is a very technical thing as per the guidelines and protocols that are followed,” he said, asking people to remain cautious.
“Around 77% of people are still vulnerable and we cannot be complacent. We should be serious about maintaining social distance, wearing masks and washing our hands regularly.”
According to Jain, the protocol for the new surveys is still being finalised. “We are chalking out a strategy. It will be spread across the city, cover both containment zones and normal areas, include individuals of different age groups and the sample size will not be less than the previous survey,” he said.
Officials in the health department, who asked not to be named, said the survey will be carried out by district surveillance teams and health department officials between the 1st and 5th of every month.
The teams will fan out across neighbourhoods from where they will pick out people according to the stratification they need to follow, varying samples by age, gender and locality to represent as random a set as possible, this person said. The tests will be done through the Covid Kavach kit developed by Gujarat-based Zydus Cadila.
NCDC, which carried out the previous survey on behalf of the Delhi government, is unlikely to be involved this time. The local teams carrying out the process will report to the district magistrates.
“The sero surveys need to be repeated periodically to be able to establish a trend with respect to the disease spread. When ICMR approved antibody (blood) testing, it had advised all states to conduct sero-surveillance using the test from time to time to get a sense of how the disease has spread within the community. This is to be done by states and the Centre will pitch in if they require assistance, especially those with high disease burden,” said a Union health ministry official, requesting anonymity.
Dr Lalit Kant, the former, former head of epidemiology and communicable diseases department in the Indian Council of Medical Research (ICMR), said: “Sero-survey done periodically can help the government focus their efforts – both in terms of speed and scale – in protecting the population still vulnerable to exposure to the Sars-CoV-2 virus.”
“As far as the sample size is concerned, it is determined through a formula that primarily depends on prevalence rate of the disease. In Delhi, the prevalence rate has gone up, so the sample size for the upcoming survey is likely to be higher,” he added.
Experts cautioned that Covid-19 still is scantly understood, especially with how it can lead to no symptoms in large proportions of the infected population, and that antibody testing in itself was a complicated process due to the way the virus behaves.
“We are dealing with a new virus here and there is a lot of unknown. For instance, at this juncture, we do not know how long antibodies would stay in the human body after recovery from Covid. That is what makes trend analysis important,” said Puneet Misra, professor of community medicines in the All India Institute of Medical Sciences (AIIMS) Delhi.
“Methodology and sample selection will play a crucial role in the upcoming sero-surveys and the government should consider involving experts and specialists in this regard,” he added.
(With inputs from Rhythma Kaul)