A paper was just published yesterday in Science presenting results from a contact tracing study in two states in southern India that entails the largest contact tracing study in the world to date. This study traced contacts from 84,965 confirmed cases, 575,071 in total, and revealed a number of interesting insights about transmission and risk, confirming many known principles and shedding light on important understudied questions such as transmission rate among children.
Here are a few of their key findings:
A small number of cases produced a high percentage of the spread. >70% of infected cases had none of their contacts test positive, while 8% of the infected cases produced >80% of the contacts testing positive. This affirms the notion that this virus tends to spread through super-spread events, and better characterization of individuals and settings prone to spread are important.
Transmission within households (9%) was much higher than at healthcare settings (1.2%) or in the community (2.6%).
High risk travel exposures, involving close proximity to infected individual for >6hr, led to a secondary attack rate of 79.3%, affirming the notion that time of exposure and extended contact in enclosed indoor settings are crucial factors increasing risk of transmission. The study doesn't specifically say, but it seems like most of this travel would have occurred on trains and buses, which might entail crowded, enclosed settings with poor ventilation, the worst environment in terms of super spread risk.
The highest probability of transmission occurred from individuals in the same age group, especially in children, demonstrating that children were primarily being infected from other children. This study made it clear that indeed children can spread the virus to others, and in fact quite efficiently.
The age group with the highest transmission rate was the young adult group ages 20-44yr.
Unsurprisingly, they found that government lockdowns clearly had an effect to substantially reduce the level of transmission.
They found an overall case mortality rate of 2.1%, and confirmed the variability over age and pre-existing conditions, with older individuals and/or with diabetes, hypertension, liver disease or renal disease comprising the highest risk of death.
As found other places, the risk of death among children was low (0.05%, 1/2000) and among elderly very high (16.6% for >85).
As has been found in the USA, the death rate has decreased over time, with individuals infected in July having 13% lower death rate than those infected in May or June. The reasons for this decreasing death rate is not clear.
This Indian study affirms many of the principles we have learned about how SARS-CoV-2 spreads and what groups are at greatest risk of death from COVID-19, with many results consistent with studies conducted in China, Europe, and the USA and demonstrating shared transmission characteristics in different demographic, cultural, and socioeconomic settings.
Two new results that seem most notable is the reduction in mortality in the summer and spread among children.
The reduction in mortality for summer infections relative to spring infections is interesting, although it is not clear whether this has anything to do with viralence of the virus, or is just a function of more effective treatment or lower inoculant levels in the heat of the summer.
Also, this study importantly affirmed that children indeed can spread the virus, quite efficiently, and in the social setting of India most children were affected by other children. This should dispel the notion that we can ignore the possibility of child-to-child transmission.