What explains the second wave of COVID-19 cases in India?

Part 1: Overview and the Amravati Outbreak

India is experiencing a second wave of SARS-CoV-2 cases.  This started in a few places such as Amravati District (Maharashtra) in February.  But many more places saw spikes in March. Since the end of March a large number of cases and deaths have been recorded and reported in most states across the country with the official daily case counts exceeding that of the first wave.  

Figure 1. Daily new cases in India.

We critically examine three explanations for this second wave.  One is that the spike started in Amravati District in eastern Maharashtra state and is spreading across the country.  This is really an explanation about where the second wave started, not why it happened.  

A second explanation is that the spike is a behavioral response, perhaps in response to the country’s vaccination campaign or to migration or just COVID fatigue setting in or a false confidence setting in seeing the decrease in number of cases.  As people saw vaccinations begin, perhaps they felt it was safer to interact with others, even if they had not personally been vaccinated.  Moreover, seasonal migration -- or migration to vote in rural elections -- may have picked up during this time.  This increased activity is leading to more infections.  

A related explanation is that certain pockets of society, perhaps high socioeconomic status individuals, have been more cautious than other parts of society. They were spared the first wave because they were more adherent to social distancing rules.  But their compliance has waned and this new surge is due to the introduction of this pocket of susceptible persons to the ecosystem. This is a variant of the behavioral explanation because it focuses on reduced compliance with social distancing, but among a subset of the population.  

A third explanation is there are new variants of COVID-19 and these are more transmissible.  The origin could be foreign (South Africa, UK, or Brazil) or could be a unique Indian strain (B.1.617).  A related explanation is that a significant portion of India’s population escaped the first wave without significant harm because they had cross-immunity from a prior coronavirus.  However, the cross-immunity is somewhat less protective against new variants because they are a bit more genetically distant from prior coronaviruses.  

These three explanations are not mutually exclusive -- they could all be true to some extent.  Moreover, they are not the only explanations.  But they do have different implications for policy.  For example, if the explanation is that optimism about vaccination is causing people to take risk, then a communications campaign that dampens expectations about vaccination may induce people to be more cautious in their daily lives.  If the problem is a COVID-19 variant, then the answer may be to accelerate vaccination -- assuming that vaccines are effective against the new strain.  

In this and subsequent posts, we’ll explore each of these explanations, starting with the Amravati outbreak.

I. The Amravati Origin Story

Let’s start with the Amravati origin story. During the start of the current surge, Amravati and Yavatmal had nearly 50% test positivity rates and were regarded the epicentres of the virus ([1]). Figure 1 plotts the reproductive rate in February and March for districts surrounding Amravati, as well as some that are more distant.  We have indicated which are in other states and which districts are connected with Maharashtra train lines (blue lines).  There is some evidence for the Amravati origin: Amravati seemed to have experienced a surge one week before any other districts and districts that are closer Amravati see a surge that is a bit earlier than districts far away from it.

But it is not an open and shut case.  Thane and Pune are not directly connected to Amravati, and yet both experienced a surge before places (like Nashik) in between them and Amravati.  Perhaps Thane or Pune had its own independent outbreak but Amaravati caught everyone’s attention because it happened a week earlier.  Or perhaps people from Amravati did not stop in Nashik as much as they did in Jalgaon, Thane, and Pune.

Even if Amravati was the cause of the subsequent outbreaks, it was only one week ahead and  we don’t (yet) have a theory for why the outbreak started there.  The Amravati theory offers a proximate cause, but not a root cause of the recent spike throughout India.  

One positive implication of the Amravati origin theory, however, is that the outbreak in Amravati was short-lived.  The outbreak lasted only 3 weeks.  After that the city locked down and the outbreak subsided.  The Amaravati division has partially relaxed its lockdown and cases did not spike back up.  One hopes that the same pattern will be repeated elsewhere.  But a warning is due: although the figure above seems to show a decline in reproductive rate by April 1 in all districts, that does not mean that rate actually fell.  Our graphs use the latest available data and that is usually missing all the positive tests on those dates because those have not all been tabulated; in time they will be added back in via data revisions.