Those are the words of Pliny the Elder (except for the COVID part). Coincidentally, he died while trying to save friends during the eruption of Mt. Vesuvius. As it turns out, today's home is also where COVID-19 comes to visit, brought in by household members.
One thing we are sure of in speaking about COVID-19 is that it is transmitted person-to-person.  Our homes are not exactly super-spreader sites, but once COVID-19 enters, it will have an opportunity to infect all of those in the home. This homegrown transmission has been especially true in homes more densely populated – small homes, lots of family, or even nursing homes. Transmission of COVID-19 within homes is increasingly important as the school year continues; any parent will tell you that children in elementary school bring home lots more than their book bag and homework.
Using the Swedish National Vaccination Register, researchers identified 1.6 million families , about 2.9 million individuals. The defined immunity as having received the entire course of either the mRNA vaccines, AstraZeneca’s vaccine, or having had COVID-19. They looked at subsequent infections  in the household between April and May of this year.
Most households, around 60%, consisted of two adults. The nonimmunized were somewhat older, 57 vs. 27, with fewer medical problems, less income, and fewer native-born Swedes than the immunized within households.
5.7% of the nonimmunized developed COVID-19 during follow-up. The graph demonstrates a clear dose-response to the presence of immunized individuals within the household. The more household members immunized, the smaller the risk of infection among the nonimmunized. Vaccination protected not only the vaccinated but their family, household members. One immunized member of the household reduced the risk for the nonimmunized by roughly half. In those larger families of five, three members vaccinated reduced the likelihood of infection by nearly 94%.
Hospitalization for COVID-19, defined as severe infections, occurred in roughly 12% of those infected. Again, an immunized individual within the household had a dose-response impact on hospitalization – reducing that risk by half or more, mirroring the reduction in infections.
“These findings suggest that vaccines [and infection-acquired immunity] are associated with a reduction in the transmission of the SARS-CoV-2 virus within families, which likely has implications for herd immunity and pandemic control.”
Several findings are important. First, immunization can come from prior infection, just as it does from vaccination. I will not recount again why the infectious route is more dangerous.  Second, immunity reduces the spread and severity of COVID-19 in our most intimate communities (herds?), our households. Immunized parents protect children, immunized children protect parents and their siblings – you can protect Granny by your immunity to COVID-19. Third, even partial immunity, the first dose in the vaccination series, conferred immunity to the household.
The findings apply to COVID-19’s alpha variant, but we can generalize that they should be valid for the Delta variant. Because the Delta variant is more infectious, it is also likely that partial immunization, only one dose of the vaccines, will be less protective of yourself and family members than the complete treatment.
 Pliny the Elder also said, "In these matters the only certainty is that nothing is certain."
 Defined as individuals living in the same home address
 95% of subsequent new cases of COVID-19 were PCR+.
 It is also more problematic because you need to show COVID-19 infected you with a PCR test.
Source: Association Between Risk of COVID-19 Infection in Nonimmune Individuals and COVID-19 Immunity in Their Family Members JAMA Internal Medicine DOI: 10.1001/jamainternmed.2021.5814