The Covid-19 lockdown logic and science behind it

As India goes under a lockdown with new coronavirus cases becoming a steady stream, it’s important to assess the role that lockdowns and strict social distancing can play in controlling the outbreak.

The objective of any lockdown or social distancing mechanism is the reduction in the rate of infection. Health experts say that social distancing is the best way to prevent hundreds of thousands of cases appearing in batches, overwhelming a country’s health care system.

When unchecked, the overwhelming nature of Covid-19 was seen in the initial outbreak of the disease in China’s Hubei province through January and February, in Italy through the end of February and early March, and New York state most recently. In all three cases, governments eventually resorted to lockdowns with varied levels of success.

What is the science behind lockdowns?

TARGET R0<1

Any lockdown targets a reduction in the basic reproduction rate of an infection or to reduce the number of people each confirmed case infects, according to a study published by the Imperial College London Covid-19 Response Team on March 16.

Scientists use a basic reproduction number, R0 or ‘R naught’, to indicate how contagious a disease is. R0 is the number of people who can contract a disease from one infected person. The R0 of Covid-19 remains a matter of research, but a study titled ‘Early Transmission Dynamics in Wuhan, China, of Novel Coronavirus–Infected Pneumonia’ published in the New England Journal of Medicine, determined a baseline assumption of R0=2.4.

The larger goal for any government is to keep R0 below one – that is each confirmed case infects fewer than one person.

Here’s what happens with various scenarios.

1: NO CURBS

In this case people are free to move around and interact. Based on R0=2.4, research in Wuhan finds that each infected person will infect 2-3 people on average.

As the first case is reported, the infected person passes it on to a second wave of two or three more people who are also moving without restrictions. The newly infected people again infect the same number of people creating a cycle. This leads to a very abrupt rise of cases as seen in both China and the US before any lockdowns were placed.

Given an estimated R0 of 2.4, researchers at Imperial College London predicted 81% of the population would be infected over the course of the epidemic. The biggest issue was that most of these cases occurred at the same time, causing a spike in the epidemic curve, overburdening health care systems.

2: LOCATION-WISE LOCKDOWN

In this case, a region with a high number of cases is locked down -- as in China’s Hubei .

In a study titled ‘The effect of travel restrictions on the spread of the 2019 novel coronavirus (COVID-19) outbreak’ published in the journal Science by a group of American, Italian and Chinese researchers on March 6, scientists found that travel restrictions in Wuhan slowed the spread. While the model output showed no noticeable differences in the epidemic in Wuhan, it showed a delay of about three days for other locations in China.

This strategy focuses on slowing down, not necessarily suppressing the epidemic. This flattens the curve slightly, buying time to strengthen health care facilities. It also means a reduction in peak health care demand, freeing up hospital beds and protecting those most at risk.

3 : LOCKDOWN AND HOME-QUARANTINE

In this case, extensive social distancing is applied throughout the populace by limiting people’s movement through both home-quarantines and lockdowns over a period of time.

The aim is to reduce R0, to below 1 and take case numbers to low levels or eliminate human-to-human transmission.

In the Imperial College model, this required social distancing for the entire population, over a long period of time.

“It is likely such measures will need to be in place for many months, perhaps until a vaccine becomes available,” one of the study’s researchers, Neil Ferguson, said in a written statement issued by the college. “The effects on countries and the world will be profound.”

This strategy yields the best results so far as the virus spread ends up being a trickle.

By closely monitoring rise in the infections, governments may be able to lift curbs temporarily, and reinstate them if numbers began to rise.

This is the kind of lockdown India has enforced for three weeks.