I got the Pfizer vaccine and now in a trial to see what adding the Moderna one will produce. Have been living with my Covid positive brother since Wednesday and I’m still negative. We have been living together as normal, he’s not been isolated from me. He does have zero symptoms though.
A lot of larger studies suggest household attack rate is 10-15% from symptomatic and 1% from asymptomatic case. There is increasing convergence towards these numbers as time goes on which suggests that it is probably right.
It gets more complex when you calculate what is the probability of someone with a positive PCR test being actually infectious, without knowing the Ct it is roughly 33% to 50% at the highest. Given overall coronavirus prevalence is waning then the chance of a PCR positive being infectious is very low indeed.
but lets go with PCR positive = infected and infectious and so your brother is infected and infectious the index case for the experiment.
Which gives you a no vaccine baseline, overall you would only expect one in 100 household contacts to be infected when the index case is asymptomatic. and one in 8 or one in 10 from where the index case is symptomatic. I haven't bothered to do the math exactly because it isn't neccessary to get a feel for where the approximate numbers are.
Without being vaccinated, you would have to repeat this exposure experiment almost 20 times to be get just 18% (1-0.99^20) chance of being infected by household contact with an asymptomatic index case, your brother. Your probability would be 82% of not being infected even with no vaccine. Doing the same experiment 100 times would only give you a 40% chance of being infected by your asymptomatic brother, who might object to being repeatedly infected, 100 times or more to do this experiment which would also take 2 or 3 years to do.
If the vaccine does exactly as the more optimistic and less solidly supported claims suggest and halves the rate of infection then the overall attack rate should be <0.5% from an asymptomatic index in households if everyone else is vaccinated, and it should be 5% to 7.5% from symptomatic index cases and in all situations the proportion of symptomatic infection should be 1/10th of all infections (which is the only relatively solid data point relating to the Pfizer vaccine in all of this)
Being vaccinated you would have to do this 20 times to get 9.4% chance of being infected if the vaccine is 50% effective at preventing all infections. So your brother might not appreciate this experiment at all.
Using yourself as a data point to determine a vaccine effect it is unfortunately beyond anecdotal. You can ascribe not being infected to being due to a vaccine effect but it is just as reasonable to say it just as due to transmission itself, in all this situation vaccine or not, is rather unlikely. It is worth thinking about what things mean in absolute terms.
What is also likely is contact and antigen exposure will boost the initial vaccine antibody titer and confound your data in the moderna booster trial, congrats you could be the high titer outlier that skews the data in the trial. The same effect was seen in the single dose trials, previously infected people had so much higher titers on single vaccine dose that it was sufficient to skew the single dose average antibody titer well above that of unvaccinated recovered people, but because it made the results look better than they were all the experts conveniently overlooked this.
science sucks sometimes.