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We need to limit the number of close contacts we have each day and each week. SARS-CoV-2 transmits through prolonged close contact. What do we mean by close? The virus is transmitted through direct physical contact or over short distances by expelled respiratory droplets. 1/17
Long-range transmission (beyond 1-2m) by aerosols is relatively rare, happens in specific circumstances, and does not appear to be a major mode of transmission. 2/17
Close means close. The risk of transmission risk is minimal at 2m, low at 1m, and increases rapidly if we get closer, or touch each other, or touch contaminated surfaces. 3/17
The risk is increased when crowded together, and by loud conversation, shouting and singing, especially indoors; but crowding, contact and raised voices are risky outdoors too. The risk is reduced by good hygiene, face coverings, staying seated, and quiet conversation. 4/17
If we mix with a lot of people in a large group, we offer the virus many more opportunities to transmit than if we are strictly segregated into smaller groups. 5/17
We have fewer close contacts than we might think. The classic POLYMOD study suggests that younger people have 14-18 close contacts per day (school, social and home), and working adults 12-14 contacts. 6/17
These are not casual contacts, they are close enough to have a significant probability of viral transmission. A schoolgoer will have 20-30 social and conversational contacts per day, but only half of these will involve close physical proximity or contact. 7/17
This fits with what we know about the dynamics of viral transmission. The basic reproduction number (R0) for SARS-CoV-2, how quickly it spread before we took physical distancing and hygiene precautions, is estimated at between 3 and 5. 8/17
So, before we took precautions, each infected person went on to infect between 3 and 5 other people. 9/17
In early March 2020, if a person infected with SARS-CoV-2 had about 10 close contacts per day, was infectious for about 8 days: that’s 80 close contact events, and yet on average only 4 of those contacts resulted in the transmission of the virus. 10/17
So even for close contacts, the risk of infection per contact is 5-10%. If you are with the same person day after day the risk is cumulative, so household attack rates are higher at 20-50%. 11/17
This, combined with the evidence that children are less efficient at transmitting the virus than adults, explains why schools are not a major locus or driver of COVID-19 12/17
This is the definition of ‘close contact’ we use to analyse the spread of disease. When public health specialists investigate an outbreak, they use a wider definition – they cast the net wider to test anyone they suspect might have been exposed 13/17
This virus is not particularly good at transmitting, but it’s good enough to create an epidemic that grows exponentially if we do not limit our contacts, and reduce the risk that any close contact leads to infection. 14/17
The good news is that of we do reduce our close contacts to 20-25% of what they would have been before COVID-19, and take precautions during such contacts, we can minimize the risk of transmission and suppress the virus. 15/17
The good news is that there are a lot of things we can do safely: smaller groups, physical distance, hand hygiene, respiratory etiquette, quiet conversation, face coverings if necessary, stay home if symptomatic 16/17
The bad news is that because we need to mix between households for work and education, we will not be able to congregate and mix in large groups for other purposes for a good while yet. We need to protect our priorities. 17/17
