Question: My sister and her family traveled to visit her in-laws down south (via flight, several airports) and are headed our way later this week to hang out for a day.  Both she and her husband are fully vaxxed.  They are coming with my two nieces (ages 8 and 10).  How dangerous is it to have my unvaxxed kids playing outside with her unvaxxed kids for the day? Our main driver of concern is that our nieces will have been in multiple airports over spring break season.

Answer: Individual-level risk assessments are difficult because of all the factors at play among the [potentially] infected host, the environment, and the susceptible host (see Q&A of 11/16).  I’ve attempted to break down these factors a bit below.  There’s a lot you can do to minimize risk.  Here are a few suggestions: a) keep the kids’ interactions outdoors, like you plan; b) have the kids keep their masks on; c) try to ensure good mask fit for all four kids; d) when the kids need to eat or the like, have them keep ample distance between themselves; e) try to minimize the kids’ shouting, especially at close-distance.  It might also help assuage concerns if you talk with your sister about the public health practices they follow(ed) on their trip.   

1. Your nieces’ potential exposure: They’ve been traveling, visiting family, and moving through 2+ airports.  That’s a lot of potential exposure opportunities.  Have they been consistently wearing their masks?  Keeping their distance?  Washing their hands?  Clearly, these behaviors reduce potential exposure.  Did they get tested before flying?  Having a negative test won’t remove all possibility of infection, but it will also assuage some concern.  Was the extended family also vaccinated? With vaccination limiting infection risk, your nieces’ potential exposure is also reduced. 
2. Your nieces’ potential infectivity: If your nieces were infected, then we’d want to understand their potential infectivity– droplets expelled, viral particles per droplet, fraction of droplets that make it past the facemask, etc.  Here, we know that infected children have similar viral loads as adolescents and adults, however, the overall infectivity of children is less than adults with a recent PLOS study estimating children’s infectivity to be 63% (95% CI: [37%, 88%]) relative to that of adults.   
3. The environment: We know that COVID transmits far more easily in closed spaces, crowded places, and close-contact settings.  Keeping the children outdoors will greatly minimize risk as any viral droplets/aerosols will be quickly dispersed.     
4. Your kids’ susceptibility:  Here we’re thinking about things like the fraction of droplets inhaled, your kids’ underlying health, duration of exposure, and mask use/fit.  All of these factors influence your children’s susceptibility.  I will note that a recent study found that “the susceptibility of children (under 20 years old) is 43% (95% CI: [31%, 55%]) of the susceptibility of adults.”