Science published a paper Wednesday based on one of the world’s largest contact tracing exercises. The study, covering 85,000 primary cases, was conducted in Andhra Pradesh and Tamil Nadu by a team of researchers led by Ramanan Laxminarayan of the Center for Disease Dynamics, Economics and Policy. The study tracked the spread (or no spread) of the infection in around 575,000 contacts out of the 85,000 to come up with some really interesting findings. HT reported those findings Thursday. Still, given the magnitude of the exercise, I would take any study with a sample as large as this seriously, and the fact that it answers some very basic questions about coronavirus disease (Covid-19), some of the findings are valid. repeat in this office.
1) Not all people infected with the Sars-CoV2 virus, which causes Covid-19, transmit it
We have known this for some time, but anecdotally. Previous installments in this column have addressed the fact that not everyone who comes into contact with an infected person becomes infected (or not all infected people transmit the infection – essentially two ways of viewing the same event), and how the most infections are caused by so-called super-spreaders. The Science study quantifies them. For example, it found that 70.7% of the 85,000 index patients (or primary cases) did not transmit it to any of their contacts, including close ones who live in the same household. The reasons for this are not known.
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2) Some of those infected with the Sars-Cov2 virus transmit it to too many people
These are the super-spreaders mentioned above, and the study found that 8% of index patients were responsible for 60% of secondary infections. This means that around one in 12 people infected with the disease have a chance of becoming a super-spreader (we still don’t know why, although it is likely a combination of physiological, social and environmental factors). If only there was some way to identify possible superprocessors …
But this is where the study starts to get interesting. We have always known that there are risks when coming into contact with an infected person or sitting next to one on a long flight. The Science study also quantifies them. This is the type of information that actuaries could use to construct a risk matrix for daily activities.
3) The secondary attack rate (an index patient transmitting the infection to a contact) is 10.7%
If we were playing in danger, the question to this answer would be: What is the risk of a person contracting Covid-19 when exposed to someone who is already infected with the virus? And this also occurs in the case of “high-risk contacts”, according to the study: people who have had close physical and social contact with the infected person, and without masks or other protective measures. For low-risk contacts, this probability is 4.7%.
4) The secondary attack rate is the highest for those sharing transportation with an index patient for at least six hours: 79.3%
This makes train and air travel (medium duration) the riskiest.
Most daily interactions outside the home (even in the workplace) fall within the study’s definition of low-risk criteria, something that should reassure those of us who have already returned to the offices. Interestingly, while studying cases of similar similar infections, the researchers found that “improved transmission in similar pairs” was highest among children under 14 and people over 65, findings that could affect school-opening of strategies and also explain why Covid-19 devastated nursing homes and nursing homes in some parts of the world.
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