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(CNN)More than half of residents living in Mumbai's crowded slums may have contracted coronavirus and are likely being infected at a much higher rate than those not living in slum areas, a new study has found.
The study released Tuesday raises questions over the level of testing in India, which has the third highest number of confirmed cases in the world after the United States and Brazil.
On Wednesday, India reported it had crossed 1.5 million reported coronavirus cases after more than half a million infections were recorded in just 12 days. It took nearly six months for India to reach its first 1 million confirmed cases.
Mumbai, India's financial capital with a population of more than 12 million, has confirmed more than 110,000 cases, including at least 6,180 deaths, according to official statistics. The city is in Maharashtra, the worst-hit state in India with more than 377,000 confirmed cases and at least 14,000 deaths.
The study -- a collaboration between local authorities and medical institutions -- found that 57% of the samples collected from slum residents tested positive for coronavirus antibodies, while only 16% of those living outside of slums tested positive, according to a news release Tuesday.
The researchers used antibody tests -- which are used to test whether a person had coronavirus in the past -- to analyze more than 6,900 random samples collected from participants living in Mumbai in the first half of July.
Antibody tests, often called serologic tests, look for evidence of an immune response to infection. In theory, that should show that the person had coronavirus in the past and has since recovered, but the US Centers for Disease Control and Prevention said in May that antibodies in some people can be detected within the first week of Covid-19 onset.
Coronavirus in slums
Researchers said that the high prevalence of coronavirus in slum areas could be due to population density and common facilities, while the lower prevalence outside of slums could be due to better social-distancing and hygiene practices.
But the authors noted that a high proportion of the cases were likely asymptomatic and said there was a low rate of death among those who were infected -- perhaps as low as 0.05%.
"This could be attributed to effective containment efforts and active measures to isolate symptomatic cases by (the Mumbai city government)," the researchers said in their report.
One reason why a relatively high proportion of residents appeared to have been infected despite lockdown measures could be that people outside slum areas were dependent on those living in slums for services such as gardening, cleaning and driving, said Utture Shankar, the president of the Maharashtra Medical Council, which is advising the government on its Covid-19 efforts.
Although the samples were only taken from three wards, other parts of the city are likely to see similar results when tests in those areas are completed, Shankar said.
India has a relatively low Covid-19 mortality rate per capita compared to other hard-hit countries, with only 2.47 deaths per 100,000, compared with 45.24 in the US and 68.95 in the United Kingdom, according to Johns Hopkins University data. Experts have pointed to India's relatively young population as a possible explanation for the lower mortality rate, as the young are less susceptible to dying from coronavirus.
The potential impact of coronavirus on India's slum areas has been a concern since the pandemic began, with doctors warning that an outbreak would be unmanageable in places where residents live cheek-by-jowl in tiny homes with no running water, and share toilet facilities with several other families.
Indian officials took steps to limit the spread of coronavirus in slum areas. For example, Shankar said authorities had taken measures such as regularly cleaning bathroom facilities, and had delivered free food to slum residents in an effort to reduce the number of people going outside to eat.
As of Monday, there were 627 slums in Mumbai that were active containment zones.
India's containment efforts
Despite the growing number of confirmed cases, India is expected to further ease coronavirus restrictions in the coming days, although the details are yet to be announced.
The Indian government has highlighted its successes, with Prime Minister Narendra Modia on Monday pointing to the country's widespread testing, high recovery rate and low number of deaths compared with other countries. According to government statistics, India has carried out more than 17 million coronavirus tests as of Wednesday.
The government has also pointed to the high rate of recovery -- according to government statistics, almost 1 million people have recovered from coronavirus. In India, patients with mild and moderate symptoms are considered no longer active after 10 days of symptom onset if they meet certain conditions. A test to confirm that they no longer have the virus is not required. Severe cases can only be discharged after one negative coronavirus test.
However, the Maharashtra Medical Council's Shankar said that Tuesday's study shows that the testing had clearly not captured all the cases. Although India had done well to increase testing capacity, it needed to increase it further, he said.
Last week, antibody tests on a random sample of people in the Indian capital of New Delhi found that nearly one in four residents may have been infected with coronavirus.
Other cities around the world have conducted similar antibody studies in the past, with much lower results.
In May, Sweden said 7.3% of people in its capital Stockholm had developed the antibodies by late April, based on 1,118 blood tests carried out in one week.
In New York, a study sponsored by the New York State Department of Health found that 14% of adults in the state had Covid-19 by the end of March, 10% higher than the official count.
