Covid: Has Mumbai reached the endemicity stage?

The Covid-19 cases in Mumbai have been hovering between 400 and 500 for the past several weeks now. The number of deaths has been under 10, even as the city’s lockdown restrictions have been eased completely. Barring train travel, public movement in the city is almost back to normal. Has Mumbai achieved a state of endemicity then? The Centers for Disease Control and Prevention (CDC) defines the term endemic as “the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area”. Hindustan Times spoke to experts on Mumbai’s current status:

A complex subject

Dr Murad Banaji from the Department of Design Engineering and Mathematics, Middlesex University, London said that endemicity is a complex subject that requires consideration of multiple factors. “It is not unusual for numbers to be steady for some time,” Banaji said in an email. “We’ve seen this before, both in Mumbai and elsewhere. We also see it in models and it can occur for several reasons. For example, we see it as the infection spreads from one area to the next. We can also see it as restrictions gradually ease up and mobility increases,” he said.

The city’s ‘R’ or reproduction number has been hovering around 1 suggesting that infections are neither surging nor dying away. R refers to the average number of people a Covid positive person infects. The serosurveys carried out in Mumbai have pointed to a high level of exposure to the virus. The city’s vaccination numbers have also gathered momentum with half of the adult population fully vaccinated and nearly 93% given at least one dose.

According to Banaji, apart from the multiple factors like exposure to the infection and vaccination, the steady Covid-19 numbers in the city could be because of “endemicity” or an “endemic steady state”. “In this situation, put roughly, you have a balance of effects: (i) people becoming immune to the disease either through vaccination or after being infected, and (ii) the population losing immunity - e.g. migration of people who are susceptible to the disease, or people gradually losing immunity over time,” he said adding that the “endemic steady-state” can be stable over a longer period of time. “If so, we shouldn’t see major uncontrolled surges. We also won’t see disease dying away. So, in a sense, endemicity is both good news and bad news,” said Banaji.

So, is Mumbai in an endemic steady state? “Probably not quite - we can’t be sure,” said Banaji. “The reasons why numbers are stable could be a mixture of the factors above. But, given the very high prior infection, and decent levels of vaccination, it is possible that the city is seeing something roughly like an endemic steady state,” he said.

Past disaster situation

Epidemiologist Dr Jayaprakash Muliyil said that it is best to not label the situation with terms like endemicity. “What we know is that the city has crossed the ‘disaster situation’ and now it is in a ‘health situation’ wherein people go about their lives and are well aware of precautionary measures like masking and vaccinations,” said Muliyil. “We have also learnt that the virus and its variants respect the previous infection to a large extent. Except for the difference in infectivity, we have not seen a major difference in mortality,” he said.

According to Muliyil, even as there is no emergency situation, the government should be vigilant, and constantly tracking variants. “Surveillance is crucial,” he said.

While it is believed that the number of reinfections and breakthrough infections may not be high, there is no data available on this. “We don’t know very accurately how long immune responses last after infection/ vaccination - although there is more and more data coming out on this. If immunity is more short-lived than we think, then the population becomes vulnerable to new surges quickly,” said Banaji.

He said that the most worrying scenario would be if a new variant starts to circulate, which spreads more easily from person to person, or causes more severe disease, or is more easily able to infect people who have been infected before or vaccinated. “If this happens, then the dynamics could change rapidly and there could be a major surge,” he said.

Thick tail

A member of Maharashtra’s Covid-19 task force, Dr Shashank Joshi said that the World Health Organization will have to first declare the pandemic as over, in order to talk about endemicity. “There may be some degree of baseline endemicity-like stage in Mumbai, but it is still too soon to say,” said Joshi adding that the steady numbers seen in Mumbai are the thick tail of the second wave.