Virus World

The rate of death went down in patients over 65. Researchers also found that children of all ages became infected and spread the virus to others. With 1.3 billion people jostling for space, India has always been a hospitable environment for infectious diseases of every kind. And the coronavirus has proved to be no exception: The country now has more than six million cases, second only to the United States. An ambitious study of nearly 85,000 of those cases and nearly 600,000 of their contacts, published Wednesday in the journal Science, offers important insights not just for India, but for other low- and middle-income countries. Among the surprises: The median hospital stay before death from Covid-19, the illness caused by the coronavirus, was five days in India, compared with two weeks in the United States, possibly because of limited access to quality care. And the trend in increasing deaths with age seemed to drop off after age 65 — perhaps because Indians who live past that age tend to be relatively wealthy and have access to good health care. The contact tracing study also found that children of all ages can become infected with the coronavirus and spread it to others — offering compelling evidence on one of the most divisive questions about the virus. And the report confirmed, as other studies have, that a small number of people are responsible for seeding a vast majority of new infections.

An overwhelming majority of coronavirus cases globally have occurred in resource-poor countries, noted Joseph Lewnard, an epidemiologist at the University of California, Berkeley, who led the study. But most of the data has come from high-income countries. “It still surprises me that it took until this point for a lot of data to come out of a low- or middle-income country about the epidemiology of Covid,” he said. In particular, he added, few studies anywhere have done contact tracing at the scale of the study. “I think it’s some of the most important data we collect in an epidemic in order to decide what kinds of interactions are safe, and what kinds are not,” he said. And yet, “data like this has not really been published very much.” Though its overall total of cases is huge, the per capita number of cases reported daily in India — and in many other low-income countries, including in Africa — is lower than in Spain, France or even the United States. And its number of deaths has not yet topped 100,000 — which has surprised some scientists. India “is a place where you would expect a disease like this to roar through, at least in the older populations,” said Dr. Krutika Kuppalli, an infectious disease expert at the Medical University of South Carolina. “They haven’t seen that as much as you would expect.”

India recorded its first case of Covid-19 on Jan. 30 in an Indian citizen evacuated from China. The government began screening travelers from China and other countries on Feb. 7 and extended these efforts to travelers by sea and land on March 15. The country shut down on March 25 but reopened two months later, despite soaring rates of infection. The study focused on two southern Indian states, Andhra Pradesh and Tamil Nadu, which together have a population of about 128 million, and represent two of the five Indian states with the most cases. They also have among the most sophisticated health care systems in the country. Contact tracers reached more than three million contacts of the 435,539 cases in these two states, although this still did not represent the full set of contacts. The researchers analyzed data for the 575,071 contacts for whom test information was available...

Original study available in Science (Sept. 30, 2020):

https://doi.org/10.1126/science.abd7672

Preventing big clusters of cases would help curb the pandemic, scientists say. When 61 people met for a choir practice in a church in Mount Vernon, Washington, on 10 March, everything seemed normal. For 2.5 hours the chorists sang, snacked on cookies and oranges, and sang some more. But one of them had been suffering for 3 days from what felt like a cold—and turned out to be COVID-19. In the following weeks, 53 choir members got sick, three were hospitalized, and two died, according to a 12 May report by the U.S. Centers for Disease Control and Prevention (CDC) that meticulously reconstructed the tragedy.

Many similar “superspreading events” have occurred in the COVID-19 pandemic. A database by Gwenan Knight and colleagues at the London School of Hygiene & Tropical Medicine (LSHTM) lists an outbreak in a dormitory for migrant workers in Singapore linked to almost 800 cases; 80 infections tied to live music venues in Osaka, Japan; and a cluster of 65 cases resulting from Zumba classes in South Korea. Clusters have also occurred aboard ships and at nursing homes, meatpacking plants, ski resorts, churches, restaurants, hospitals, and prisons. Sometimes a single person infects dozens of people, whereas other clusters unfold across several generations of spread, in multiple venues. Other infectious diseases also spread in clusters, and with close to 5 million reported COVID-19 cases worldwide, some big outbreaks were to be expected. But SARS-CoV-2, like two of its cousins, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), seems especially prone to attacking groups of tightly connected people while sparing others. It’s an encouraging finding, scientists say, because it suggests that restricting gatherings where superspreading is likely to occur will have a major impact on transmission, and that other restrictions—on outdoor activity, for example—might be eased....

That’s why in addition to R, scientists use a value called the dispersion factor (k), which describes how much a disease clusters. The lower k is, the more transmission comes from a small number of people. In a seminal 2005 Nature paper, Lloyd-Smith and co-authors estimated that SARS—in which superspreading played a major role—had a k of 0.16. The estimated k for MERS, which emerged in 2012, is about 0.25. In the flu pandemic of 1918, in contrast, the value was about one, indicating that clusters played less of a role.