India is four times more populous than the US, but has just 2% the number of cases and only 1.5% of the number of Covid-19 deaths. How has the country, whose per capita income is just tenth of the US, avoided being flattened by the pandemic?

There are four possible explanations:

The epidemic may have struck later than in other countries. Since epidemics have exponential growth at the start, a small delay can have massive effects on the number of cases. India’s 21-day lockdown may have successfully suppressed the epidemic. Physical distancing is one of the best ways to slow the epidemic and, if anything, many have criticized the lockdown for being too draconian in that regard. India has not been able to test enough to count all cases and deaths. Without sufficient testing, many deaths may not be labeled with Covid-19 for official statistics, leading to an underestimation of the severity of the crisis. India may have protective characteristics against Covid-19. Researchers have proposed that the low share of elderly in the population, the high temperatures and humidity in India, widespread BCG vaccination for tuberculosis, or resistance to malaria have helped India escape the brunt of the pandemic.

First, we can rule out that India is in an early stage of the pandemic. The first infected travelers from Wuhan reached Kerala, southwest India, on January 29. While it is true that Kerala acted quickly and contained a potential outbreak, more carriers reached other states by early March.

While the lockdown will certainly have an effect on the pandemic in India in the long run, there has not been enough time for its impact to be felt. So the current low level of deaths is really telling us about what happened before the lockdown began. Moreover, it is unclear that the lockdown has been total. Besides videos of migrants crowding the Delhi/Uttar Pradesh border or Bandra station in Mumbai waiting to go home and the news of the Tablighi Jamaat Markaz event in New Delhi (which led to a spike in cases), there is also Google Mobility data that backs this thesis. In addition, if India’s caseload is low because of the lockdown, it will not help when the lockdown ends.

India, like many countries, has been unable to procure enough tests. The lack of surveillance means that many deaths due to Covid-19 may not have been classified as such. A Covid-19 death typically is confirmed by a Covid RT-PCR test. Those tests are in short supply and cost INR 4500 ($60) in India. Moreover, if an individual with Covid-like symptoms dies, but a test is not performed before his death, it does not make sense to waste scarce resources on the cadaver. Nor do officials report unconfirmed, Covid-like deaths, because there are many reasons that one may die from flu-like symptoms, and officials do not want to create panic.

As a backstop, one can look at all-cause mortality. If the coronavirus has really taken off, surely we would see the impact in total deaths. Such an analysis is complicated, however, by the fact that the lockdown has led to delays in updating death records. Moreover, the lockdown has affected mortality in ways that are unrelated to Covid-19. By shutting down travel and factories, it has eliminated transport-linked deaths and deaths triggered by air pollution. Physical distancing measures may also lower deaths from influenza, just as they reduce Covid-19 deaths. So it is theoretically possible that Covid-19 has had a substantial mortality impact, but that total deaths have not risen because the lockdown reduced non-Covid deaths.

This leaves the intriguing possibility that Covid-19 will have less of an impact in India than in other countries. However, it is important to not use it as an excuse to be less vigilant. Its reproductive rate in India is around 1.8, which implies that approximately 65% percent of the population will be infected without a lockdown or vaccine. Even if India has a death rate as low as Germany’s 0.3%, two million people could die.

With that caveat, let’s consider why the death rate may be lower in India. Initial data from China suggested that the mortality rate from Covid-19 infection is much higher among the elderly: it is 14.8% for those above 80, but just 0.2% for individuals below 39. In India, only 0.8% of the population is above 80 and nearly 75% are below the age of 40. An offsetting factor, however, is the poor health of even younger Indians. The fatality rate rises by roughly 30% if a person has cardiovascular disease, diabetes, chronic respiratory disease, or hypertension. India has roughly double the rate of heart disease as Italy, and among the highest prevalence of respiratory diseases in the world. In addition, India is home to one in six people with diabetes.

Another complicating factor is that India is home to many multi-generational, joint families. This structure places elderly Indians at considerable risk from catching diseases from younger family members, especially asymptomatic ones.

A second explanation is that hotter temperatures in India may slow the virus. There is some evidence consistent with this theory. Countries with latitudes between 30-50 degrees above or below the equator, and average temperatures between 5 and 11 degrees celsius have, thus far, borne a higher burden from Covid. However, some preliminary research has suggested that India may face higher transmission rates during the monsoon, which is India’s flu season. Humidity may also play a role; studies have found varying results on its importance as a factor. In addition, a brand new study at Harvard has suggested that Covid-19 may not go away in warm weather as colds do, because significant parts of the population remain vulnerable to the virus. The weather alone is insufficient to protect India.

Other explanations center on universal BCG vaccination in India, or domestic hydroxychloroquine use to combat malaria. While it is true, for instance, that countries with BCG vaccination appear so far to have less transmission, these countries are also warmer, have younger populations, and—because they are poorer—likely tested late. So it is hard to know whether there is a true link between Covid-19 and BCG or malaria burdens. Further analysis is required, though it may not come fast enough to help in this epidemic.

Ultimately, India’s relatively light exposure to Covid-19 remains a puzzle. It may have certain characteristics that protect it from the deadliest impact, but they do not suggest that it will escape the pandemic unscathed. Great care and vigilance are still necessary.

Anup Malani is a professor at the University of Chicago Law School and Pritzker School of Medicine; Arpit Gupta is an assistant professor of finance at the NYU Stern School of Business; and Reuben Abraham is CEO of the IDFC Foundation and IDFC Institute in Mumbai.