09/22/2020
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We've learned a lot about transmission dynamics & how COVID spreads over last 6 mnths. Here's some key insights from a recent evidence review (Cevik et. al, 2020).
Transmission risk is multi-dimsnional & depends on many factors: contact pattern (duration, proximity, activity), individual factors, environment (e.g. outdoor vs. indoor) & socioeconomic factors (i.e. crowded housing, job insecurity, higher immigrant poplns). Fig1
Sustained close contact drives the majority of infections. E.g. close family/friend contacts & gatherings are at > risk for transmission than shopping or brief community encounters. E.g. being a spouse/partner, sleeping in the same rm/space, frequent daily contact w index case, & dining in close proximity have been associated w ⬆️ risk of transmission. Longer, > frequent exposure, close proximity, # contacts, grp activities (esp. dining) = ⬆️ risk.
Many ppl either don't infect anyone or infect just 1 person, & a large # of 2° cases are caused by a small # of infected ppl. This is related to other factors as well. Individual variation in infectiousness plays a major role. Fig2
Infected ppl r very infectious for a short window, likely 1-2 days before & 5 days following symptom onset. So far, no transmission has been documented after the 1st wk of symptom onset. Fig3
While asymptomatic patients can transmit virus to others, emerging evidence suggests that asymptomatic index cases transmit to fewer 2° cases. Attack rates are highly correlated w severity of symptoms. Transmission also depends on other host factors (e.g. defense mechanisms, age, etc.)
Contact pattern is influenced by encounter setting. Studies suggest an ~20x higher risk of transmission indoors vs. outdoors. Prolonged indoor contact in a crowded & poorly ventilated envmnt significantly ⬆️ risk of transmission. ⬇️ occupancy & improving ventilation through opening windows/doors can lower risk.
Largest outbreaks around 🌍 reported in LTC facilities, nursing homes, homeless shelters, prisons, & meat plants.
Disproportionate impact on households living in poverty, racial & ethnic disparities observed (often structural) emphasize need to u