Question:  A friend of mine told me yesterday that her mom (age 75, but generally healthy) had covid last month (confirmed with 2 trips to the dr, a test and lost sense of taste as well as fever and lethargy, but recovered after 2 weeks with no remaining symptoms), but that her dad, who slept in the same bed with her mom every night and took care of her mom, never got it (he was tested twice). How can someone that close to someone sick not get it?

Answer: Great question that scientists are still trying to answer!  Short answer== The odds that your friend’s dad would have contracted COVID are high, but infection is not a certainty.  

What are the odds of infection? As stated above, we don’t fully know the odds, but they are high, especially given the circumstances you describe.  Recent findings published in CDC’s MMWR, Transmission of SARS-COV-2 Infections in Households, conclude that “transmission of SARS-CoV-2 among household members was frequent from either children or adults” with the secondary infection rate among household members ages 50+ being 62% (95% CI: 44%-77%).  Restated, this study found that 3 in 5 household members ages 50+ contracted COVID from their household member.  Now, these data aren’t meant to be extrapolated to the broader population, but I’m sharing them here to reinforce the point that the odds are high.  A paper published last month in Physics of Fluid aims to mathematically model infection risk and numerous other scientists are working on similar endeavors.  

Why isn’t infection a certainty? When it comes to COVID, there are many factors influencing transmission (Figure 1).  The three main buckets are: 1) amount of virus expelled by the infected host; 2) environmental factors that allow the virus to transport to susceptible host; 3) amount of virus inhaled by susceptible host.  But what influences these pathways, especially the infected host factors and susceptible host factors, is still not fully known.  When it comes to infected host:

“A growing number of studies estimate that a majority of infected people may not infect a single other person. A recent paper found that in Hong Kong, which had extensive testing and contact tracing, about 19 percent of cases were responsible for 80 percent of transmission, while 69 percent of cases did not infect another person. This finding is not rare: Multiple studies from the beginning have suggested that as few as 10 to 20 percent of infected people may be responsible for as much as 80 to 90 percent of transmission, and that many people barely transmit it.” -Zeynep Tufekci, The Atlantic

As described in a PBS report last month:

“‘It is clear that there are some people who, because of their own personal biology — we don’t know enough about it yet — they produce a higher percentage of aerosols versus respiratory droplets’ [said Dr. Crystal Watson, a public health preparedness expert at the Johns Hopkins Center for Health Security] …. ‘One possibility is that different individuals have respiratory fluid with different viscosity and interfacial tension, both of which affect the dynamics of droplet formation.’ [said Professor William Ristenpart, a chemical engineer at University of California, Davis]”

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