Question: My fully vaccinated husband just came home with COVID-like symptoms and the rapid test says he’s positive.  I’m also fully vaccinated, and haven’t received a booster dose yet.  What are the chances that I experience a breakthrough infection?  

Answer: Recent research findings (described below) indicate that among household contacts, 1 in 4 will experience a breakthrough infection.  The overall odds are rather high, but your individual risk will depend on a number of factors, including the amount of virus expelled by your husband, environmental factors that allow the virus to transport to you, and the amount of virus you inhale (Figure 1).  To reduce your risk of infection: ask your husband to isolate– as possible, sleep in separate rooms, eat apart from one another, use separate bathrooms; both of you wear your masks as much as possible, always when you are in the same room; and keep fresh air circulating.    

Recent research: Just four days ago, Lancet Infectious Diseases published a paper that examined this very question, Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study.  The study followed 205 household contacts of people who tested positive with mild or asymptomatic cases of COVID caused by the Delta variant.  Of these contacts, 53 tested positive for COVID (25% of fully vaccinated contacts and 38% of unvaccinated contacts).  Researchers found that: a) risk of infection was lower among vaccinated household contacts as compared with unvaccinated household contacts; 2) risk of infection was substantial among vaccinated household contacts; 3) risk of infection among vaccinated household contacts increased with time since vaccination; 4) viral load peaks were similar among vaccinated and unvaccinated individuals; and 5) viral load declined more rapidly among vaccinated individuals.  If you’d like to read more, The BMJ ran a nice article further describing the meaning of the findings, Covid-19: One in four vaccinated people living in households with a covid-19 case become infected, study finds, a quick and helpful read. 

Figure 1. Factors influencing COVID transmission/infection (from Johns Hopkins)