The second wave of the pandemic saw a sudden spurt in the number of cases; it also saw a steep plunge after it hit the peak. The fluctuations were erratic. Test positivity rate (TPR) crossed 1% mark on March 20 but in little over a month, it peaked to 36% on April 22. Then, by May 31, the TPR had dropped below 1% mark to 0.99%.
Delhi’s was not an exception. Experts said a similar trend was witnessed in some other States and attribute it to the type of virus variant as well as people’s behavior patterns. However, they were also split on the issue of herd immunity and whether there can be a third wave in the city.
A woman getting vaccinated at a centre in Delhi.
Jayaprakash Muliyil, an epidemiologist and former principal of Christian Medical College in Vellore, said whenever there is a sharp rise in cases, there will be a sharp drop in the number of cases. It was “expected”.
"Second wave started at a time when people lost patience and started venturing out. There were Alpha and Delta variants at that time too. The Delta variant is more contagious and if one person gets infected, in most cases, the whole family will get infected. This was the reason for the sharp rise. Most parts of Delhi are a city and this also contributed to it,” Dr. Muliyil said.
Susceptibility factor
After the rise, the cases started dropping suddenly. “Practically, everyone who got infected during the second wave was those who d id not get infected in the first wave. During the first wave everyone was susceptible. But during the second, after a point the virus did not have enough susceptible people. That led to the sudden fall,” he reasoned.
On the other hand, wherever the spike was not sharp, rather steady, it will take time for the cases to reduce. “For example, in Kerala there are still about 10,000 cases every day as they did not see a sharp spike in cases like Delhi,” he said.
Nandini Sharma, Head of Community Medicine Department of Maulana Azad Medical College in Delhi, attributed the rise to two reasons. First and foremost, it was a more contagious variant of the virus. Secondly, the virus got a chance to spread in Delhi as people were crowding. “The fall was sharp as the virus infected almost all susceptible people,” Dr. Sharma said.
“Some other places would have a slower rise if they have a different variant of the virus and a different population density. Mumbai took almost double the time we took to reach the peak,” she added.
Dr. Sharma said that the steep rise and fall is not unusual. “This is not something new and is one of the patterns of waves of an epidemic that we teach students. There are different patterns of waves of epidemics,” she said.
NEW DELHI , 02/07/2021: RT-PCR test of covid 19 virus in progress at GTK road on a hot morning, in New Delhi on Friday Photo: SUSHIL KUMAR VERMA / The Hindu
Jugal Kishore, Head of Community Medicine Department at Safdarjung Hospital said it was “not an anomaly” as it occurs in other places too, though not everywhere.
“In U.K., there was a similar rise and fall in cases. It depends on the variants of the virus in circulation at a point in time. Even in India, the trajectory [in other places] could be different to Delhi as the dominant variants might be different,” Dr. Kishore said.
He said if the government was detecting new cases faster during the second wave, then that would have also aided in a quick fall in cases as those people will be isolated and hence would not be spreading the disease.
Immunity and third wave
The quick drop in numbers in Delhi during the second wave could be a sign of herd immunity, observed Dr. Muliyil. About the possibility of a third wave, he said: “We need to know how much percentage of the population has been infected and how much is still susceptible to the virus. For that we need a sero survey.”
As per current estimates, about 86% of people should have immunity by natural infection in cities to get herd immunity and the figure is 60% in rural areas, he said, adding that the immunity from COVID-19 infection is “long lasting”.
However, Dr. Sharma said it is difficult to ascertain whether Delhi has attained herd immunity or not. “Assuming there is no new variant that can bypass immunity and infect people who have been already infected, we will attain herd immunity if 75-80% of the population has developed antibodies against COVID-19. Having said that, since it is a new virus, we are still learning about it,” Dr. Sharma said.
RT-PCR test under way at GTK Road in the Capital.
Herd immunity is different for different diseases. For instance, 95% of the children have to be vaccinated to attain herd immunity in case of measles, she pointed out. Currently, there are fewer number of reported re-infections and the emphasis should be on vaccinating more people, she added.
Dr. Kishore has a different take on the third wave. “Only people who have not been infected by t he virus are susceptible to it. In Delhi, majority have been infected. Also, till now there is no proof of any variant causing large scale re-infection. So, I don’t think there will be a third wave in Delhi,” Dr. Kishore said.
Less probability
But it cannot be ruled out altogether. “A third wave is possible, if the virus mutates and it is able to infect people who have been already infected,” he said, adding that people who have developed natural immunity from COVID-19 have less chances of getting infected again, compared to those getting infected after taking the vaccine.
Attaining herd immunity for COVID-19, though, is more complex than initial assumptions. “In the beginning, we thought that if 60% of the population gets natural immunity then we will attain herd immunity. Later, the virus became more infectious and we thought that 80% of the population has to get natural immunity. But now we think that even if 80% of the population have natural immunity, the rest could be infected if a more infectious variant comes,” he said.
