estimate the chance of being infected by an infected household member as 30%
Given how contagious this disease seems to be, why is this not higher? Am I misunderstanding what this is measuring? Given you are uninfected, and someone in your household is infected, you have a 1 in 3 chance of contracting Covid?
You are understanding correctly. Here are some things to keep in mind:
The reproductive number R before lockdowns was estimated at 2-3. People are infectious for 4-7 days. The average person has contact with about 10 other people daily (paper). So there could be 20-50 unique contacts over 4-7 days. Maybe 10-30 of those are higher risk contacts (long duration, close proximity). So only 5-33% of higher risk contacts are being infected (using these very rough numbers). So I’d say that Covid is not very contagious. Note that R for measles is 12-18!
There is probably some overdispersion. Say 20% of people do 60-80% of all infecting. So many people cause zero new infections. I’d presume such people just aren’t very infectious and so even if they spend a lot of time with household members they won’t infect them.
The 30% is averaging over all household members, which includes children. Children are probably less susceptible (e.g. they might have 50% lower risk of infection).
Once someone develops Covid symptoms, many households will intervene to reduce exposure. If adult children get sick, they are likely to try to isolate from their older parents.
Given how contagious this disease seems to be, why is this not higher? Am I misunderstanding what this is measuring? Given you are uninfected, and someone in your household is infected, you have a 1 in 3 chance of contracting Covid?
You are understanding correctly. Here are some things to keep in mind:
The reproductive number R before lockdowns was estimated at 2-3. People are infectious for 4-7 days. The average person has contact with about 10 other people daily (paper). So there could be 20-50 unique contacts over 4-7 days. Maybe 10-30 of those are higher risk contacts (long duration, close proximity). So only 5-33% of higher risk contacts are being infected (using these very rough numbers). So I’d say that Covid is not very contagious. Note that R for measles is 12-18!
There is probably some overdispersion. Say 20% of people do 60-80% of all infecting. So many people cause zero new infections. I’d presume such people just aren’t very infectious and so even if they spend a lot of time with household members they won’t infect them.
The 30% is averaging over all household members, which includes children. Children are probably less susceptible (e.g. they might have 50% lower risk of infection).
Once someone develops Covid symptoms, many households will intervene to reduce exposure. If adult children get sick, they are likely to try to isolate from their older parents.