What does dip in positivity rate mean?

MUMBAI: Mumbai’s daily Covid-19 test positivity rate – the percentage of tests that are positive, or TPR – has been on a steady decline, data shows. From 30% on April 4, when the city recorded the highest singleday surge of 11,206 cases, the test positivity rate came down to 10.79% on April 29, with cases dropping to 4174.

This is not an isolated case. The city’s TPR for the previous six days was respectively 12.69%, 13.19%, 13.68%, 13.75%, 14.87% and 17.26%.

What does this mean for the city that was once the epicentre of the Covid pandemic in India, and was one of the worst affected cities in the second wave that began in the second week of February and peaked two months later?

Simply put, TPR rises when most of the people who are getting tested are positive, and declines when most samples tested return negative. But this rate, experts say, has to be correlated to the number of tests. Trends across the pandemic show that when tests declined, the TPR went up. Mumbai was no different in the first wave that peaked in September 2020. The city averaged 11,175 tests per day that month, and the city’s

TPR was almost 20%. In April, though, the number of tests has averaged more than 40,000 per day, with TPR consistently declining from the second week onwards.

“A declining TPR is a good sign for Mumbai,” said Sandeep Juneja, dean of the school of technology and computer science at Tata Institute of Fundamental Research (TIFR), Mumbai. “The positivity rate goes down when the city has fewer people with Covid symptoms seeking tests. To put it simply, it means that the number of infections within the community is coming down, which is a healthy sign. It would be ideal for the TPR to drop below 5% as was the case in late January and early February.”

According to Juneja, in August-september last year, Mumbai’s TPR was around 20%. “At that time, the city was carrying out only about 10,000 to 15,000 tests each day. In comparison, the number of tests has increased to nearly 40,000 now,” he said.

From 30% on Sunday, April 4, the positivity rate fell to 25% the next Sunday on April 11. It further dropped to 23% on April 18 and to 19% on April 25. The state implemented curfew-like restrictions on April 14, and it announced stricter curbs on the movement of citizens on April 21. Civic officials said that before these restrictions came into place, they were widely testing people at crowded areas such as beaches, malls, gardens, etc.

“Since the restrictions came into place, we are carrying out targeted testing of people who have symptoms, or those who have a train or air travel scheduled,” said a civic official. “If you see the testing figures, the numbers have not fallen drastically. At this point, the reduced TPR indicates that the infection is being controlled,” she said, adding that the government will consider easing the restrictions when the rate falls below 5%.

An expert insight published by the Us-based Johns Hopkins Bloomberg School of Public Health in August 2020 has stated that “one threshold for the percent positive being “too high” is 5%”. It said that the World Health Organization recommended in May 2020 that the percent positive remain below 5% for at least two weeks before governments consider reopening.

Experts say that TPR is a more reliable indicator to ascertain whether new infections are reducing. “But interpreting it is complex,” said Dr. Om Shrivastav, member of the Maharashtra’s Covid-19 task force. “It can be well interpreted if there is consistently the same number of tests in the same geographical area or community,” he said.

According to city-based infectious disease expert Dr Tanu Singhal, the lockdown-like restrictions have played a crucial role in Mumbai’s declining positivity rate. “At the same time, people have also become more cautious, and there is definitely a change in behaviour in terms of masking, physical distancing, etc,” she said, adding that the state should not rush into easing the restrictions and instead use the time to vaccinate as many people as possible and scale up health infrastructure. “We must not forget that there can always be a third wave,” said Dr Singhal.

Does Covid-19 vaccination have a role to play in the declining test positivity rate? “It’s too premature to link them both,” said Dr. C Pramesh, director of the Tata Memorial Hospital who has been keenly tracking the Covid-19 trajectory. “Compared to our population, the number of vaccinations is still low and this immediate reduction in the test positivity rate cannot be attributed to it,” he said.

Until April 28, Mumbai had administered over 2.4 million vaccine doses, but only around 460,000 were fully vaccinated with two doses.