US study shows how domestic travel ban to check Covid can lead to more cases, cites India data

University of Chicago researchers used data from 6 countries in all to conclude that domestic travel bans of moderate duration can increase cases.

File photo of migrant workers on their way back to their native villages and cities amid the coronavirus lockdown in 2020 | Photo: Praveen Jain | ThePrint

New Delhi: Domestic travel bans to control Covid-19 transmission are likely to “counterintuitively” increase cases, especially in countries with a large migrant population, a new US study has found.

According to the study, conducted by researchers from the University of Chicago, domestic travel bans could increase cases if the duration of the restrictions is not very short or very long but is of moderate length.

“Although very short and long restrictions limit the spread of disease, moderately lengthy restrictions substantially increase infections. This suggests significant caution before imposing mobility restrictions in developing countries,” the study said.

The study, published Thursday, used data from six countries including India, China, Indonesia, the Philippines, South Africa, and Kenya and noted that such travel restrictions, which only temporarily delays the movement of people instead of preventing them completely, can lead to a surge in cases.

For instance, a large migrant population that is temporarily prevented from leaving an urban hotspot and returning to rural areas due to travel bans will eventually undertake the journey after restrictions ease.

The paper — authored by Fiona Burlig, Anant Sudarshan, Garrison Schlauch — used data on rural-urban migration, travel ban policies, and Covid-19 cases to show that these restrictions lead to a lose-lose outcome.

They cited the example of the migrant exodus that occurred in India, especially Mumbai, during the first wave, after travel bans were imposed suddenly on 25 March 2020.

“The national lockdown in the first wave trapped millions of migrants inside big cities like Mumbai that were fast-growing coronavirus hotspots. Eventually people were able to leave, but variations in government policy meant that the length of time they were detained depended on where they wanted to go,” Dr Anant Sudarshan said in a statement.

He added: “In the case of rural districts where people could return quicker – using the Shramik Special Trains for example – cases rose modestly at the destination. But for those areas where bans were in effect longer, there was a much more intensive increase in coronavirus, likely because the returning population was now more likely to be infected, having been trapped in the hotspot longer.”

How the study was conducted

The study used empirical data on the movement of migrants from Mumbai when the national lockdown was announced, and then through the three phases in which migrants were allowed movement.

The government allowed the first wave of migrants to return to their homes outside Maharashtra on 8 May, while ‘Phase 2’ migrants — returning to districts in the Mumbai Metropolitan Area — were allowed to leave on 5 June. The ‘Phase 3’ migrants, departing to all other destinations, were able to leave on 20 August.

The study estimated the impact of travel ban relaxation on Covid cases in migrants’ home locations, in each phase.

Apart from Mumbai, the paper also analysed data from five other countries where migration is prevalent — China, Indonesia, the Philippines, South Africa, and Kenya.

In all, these countries comprise roughly 40 per cent of the global population and all of them had initial outbreaks in a few hotspot locations. This was done “to study whether our estimated impacts are likely to generalize to a broader context”, the researchers said.

What the study found

According to the study, though very short and long restrictions limit the spread of disease, moderately lengthy restrictions substantially increase infections.

In Mumbai, for instance, the study found that “Phase 1 migrants have more time to spread infections, while Phase 2 migrants are more likely to be infected before returning home”. “We find that Phase 2 districts experienced more new cases per Census migrant, even in a shorter 25-day period. In this empirical setting, the lengthier travel restriction created more total infections than the shorter ban.”

In its cross-country comparison, the researchers collected subnational data on travel bans and Covid cases in Indonesia, India, South Africa, the Philippines, China, and Kenya.

The study found no evidence that Indonesia’s short travel ban significantly increased rural infections once migrants were allowed to return home.

Meanwhile, it noted that the ending of intermediate length bans in South Africa and India led to substantial surge in cases.

In the Philippines, which had a slightly longer ban duration, the study found that it led an increase in cases, but to a lesser extent than India or South Africa. It also found little evidence that Kenya’s long travel ban increased rural cases.

In China, hotspot cases had fallen more quickly than in other countries, nearing zero at the time of ban release.

“These cross-country estimates suggest that our Mumbai results, showing that the impact of travel bans on COVID-19 cases is nonlinear in their duration, generalize around the developing world,” the study said.

Exercise caution while imposing travel bans

As a result, the researchers asked policymakers to exercise caution while imposing such bans and instead recommended allowing people to move early instead of forcing them to stay.

“The evidence indicates that if the duration of the travel ban is not long enough, we may end up imposing hardship on people while perversely seeing more spread of infection. Getting the duration wrong is easy because it is not possible to predict in advance what the optimal length should be, and in a democracy such restrictions cannot be easily sustained. There may be merit in letting people go home early and indeed encouraging them to do so, rather than forcing them to stay,” Dr Fiona Burlig said in the statement.

While the study stated that domestic travel restrictions are likely to increase cases if a government is not willing to commit to a lengthy ban, it adds that such bans do help in delaying the occurrence of new cases.

“Even bans that increase cumulative infections will delay the occurrence of new cases through community spread outside hotspots. If policymakers could use this time to significantly improve the availability of healthcare, the number of serious complications and deaths may fall even as total infections rise,” the study noted.

(Edited by Rachel John)