Delhi has conducted around 100,000 rapid antigen tests for the coronavirus disease.
These tests themselves are interesting for two reasons. One, they were conducted in so-called containment zones, parts of the city roiled by the viral infection, and where there are significant restrictions on movements and activities. Two, because the aim of these tests was to see the extent of the disease’s prevalence in these hot spots (another term preferred by Indian health policymakers), none of the existing rules governing testing applied. Anyone from the area could get tested (and many did).
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Across states, even in ones such as Tamil Nadu that are testing aggressively, it isn’t usually possible for someone without symptoms, or who has not had contact with a Covid-positive person, to be tested.
The findings of the survey are also interesting. Around 7.46% of those tested returned a positive result. In research terms, this is how one can express this: in a survey of a randomly selected sample of around 100,000 people from Delhi’s containment zones, 7.46% were found to be infected with the virus that causes the coronavirus disease.
This isn’t the same as the sero (blood) survey of 20,000 people that has been launched to see if they have antibodies to the Sars-CoV-2 virus and are, therefore, potentially immune to it (at least for some time). That survey is ongoing (it will end on July 6) and the results are expected shortly after that. Between the sero survey (which also has a random sample) and the antigen tests, we will finally have a measure of the pandemic’s spread in a city that has been hit hard by it. Delhi ended Monday with 85,161 cases and 2,680 deaths, 15% of all cases in India thus far, and 15.9% of all deaths.
Hypothetically, a 7.46% prevalence sounds about right. The US CDC said last week that based on several antibody tests (the same kind of sero surveys Delhi is conducting) around the country, it believes only one in 10 infections in the country has come to light. That would put the prevalence of the disease in the US at around 7.5%. Is that a coincidence? I wouldn’t know, but sometimes diseases do tend to follow similar patterns across countries.
{{/usCountry}}Hypothetically, a 7.46% prevalence sounds about right. The US CDC said last week that based on several antibody tests (the same kind of sero surveys Delhi is conducting) around the country, it believes only one in 10 infections in the country has come to light. That would put the prevalence of the disease in the US at around 7.5%. Is that a coincidence? I wouldn’t know, but sometimes diseases do tend to follow similar patterns across countries.
{{/usCountry}}The bad news is that 7.5%, or even 10%, is far away from the 60-65% of the population that needs to be infected before we can reach herd immunity – the stage where the chain of infection gets broken because six out of 10 people the virus tries to infect have already been infected and are therefore immune. In an article in Science last week, mathematicians from the universities of Nottingham and Stockholm wrote that the number could be as low as 43%. They assumed that every infected person would infect 2.5 other people, and also made allowances for age and mobility of the population. But 7.5% or 10% is also far away from 43%.
There can be no arguing with the broader concept of herd immunity – it’s how populations survive – but, in this case, it is going to take time before 43% of the population has been infected.
Some of India’s other badly affected states and cities would do well to follow the Delhi example and test the broader population. Tamil Nadu should; as should Maharashtra. And other states, even ones that have not been hit as hard by the pandemic, should follow suit. As far back as May, the Indian Council of Medical Research recommended random sero surveys to be carried out every week, and in each one of the country’s 700+ districts. That hasn’t happened, but it’s worth remembering that states that follow this advice now may avoid the crisis that Delhi, Maharashtra and Tamil Nadu are managing with varying degrees of success.