Rise and fall in cases from time to time is a common phenomenon when an infectious disease transitions from pandemic to endemic phase, experts said on Friday as India witnesses an upward trend in Covid infections.

Underlining the current rise in coronavirus cases is so far limited to certain districts of the country, they said not wearing masks, increased travel and social interactions and low uptake of booster doses of Covid vaccine could possibly be behind the increase.

Dr Sanjay Rai, a senior epidemiologist at AIIMS, said SARS-CoV-2 is an RNA virus which has already seen over 1,000 mutations even though the number of variants of concern are only five.

Even in the case of Omicron variant, mutations happened in the receptor binding domain which increased the chances of reinfection and breakthrough infections, he told PTI.

The trend of cases rising repeatedly is a "common phenomenon" when a disease transitions from pandemic to endemic phase, said Dr Rai who is also the principal investigator of Covaxin trials for both adults and children at AIIMS.

As long as there is no increase in severity or dramatic changes in hospitalisation and death numbers, sheer increase of cases is not a cause of concern, he said.

Maharashtra, Kerala, Delhi, Karnataka, Tamil Nadu, Haryana, Uttar Pradesh, Telangana, West Bengal, Gujarat, Goa and Punjab have been recording an increase in weekly cases and positivity since June 10.

According to official sources, 51 districts in India, including 12 from Kerala, seven from Mizoram and five each from Maharashtra and Assam, are reporting a weekly Covid positivity rate of over 10 per cent.

In 53 districts, including 10 from Rajasthan and five from Delhi, the weekly positivity is between five and 10 per cent, they said.

"India witnessed a very devastating second wave of Covid infections last year which was very unfortunate. But it has led to development of a community level natural immunity against SARS-CoV-2.

"Global evidence show that natural infection provides better and longer protection against COVID-19. Also, there has been a high vaccination coverage. Hence, a severe wave in the future is unlikely until a new mutant variant is capable of invading the existing natural immunity and causing severe disease," Dr Rai added.

Dr Chandrakant Lahariya, an epidemiologist and infectious diseases physician, said SARS-CoV-2 is as much around as it was a few months ago.

"Therefore, a rise and fall in the number of infections from time to time is very much expected because that is how infectious and respiratory disease behave. Which is why every rise in cases is not a reason of concern or worry," he told PTI.

Also, a large proportion of India's adult population has received two shots of COVID-19 vaccines and a majority in all age groups are estimated to have developed immunity after natural infection, Dr Lahariya said.

So, people have developed hybrid immunity which provides protection from subsequent infections resulting in symptomatic diseases, he explained.

"Moreover, 27 months into the pandemic, tracking daily new cases is not a good parameter to assess the spread. It simply reflects that the virus is around and we already know that.

"Now is the time to track Covid-related hospitalisations and severe clinical outcome. If these parameters do not change drastically, there is no need to worry," Dr Lahariya said.

He said that whether Covid has become endemic or is still pandemic is of limited practical relevance.

"However, it is fair to conclude that COVID-19 is not a population level challenge anymore, it is more of an individual risk now. Therefore, government interventions should be more targeted at vulnerable people. It is time individuals assess their risk and determine what Covid appropriate behaviour they wish to adopt," he said.

Dr Samiran Panda, Additional Director General at ICMR, said these spike in cases remain restricted to certain pockets in districts and cannot be seen as a general rise in infections for the entire district or state.

"These occasional localised upward trend fortunately are not associated with severe disease, hospitalisations and deaths.

However, use of masks would offer protection not only against Covid but also from other diseases such as tuberculosis as well as air pollution," Dr Panda said.