Virus World

Vaccination and Household Transmission of SARS-CoV-2 In this study involving household contacts of persons with laboratory-confirmed Covid-19, the risk of household transmission was 40 to 50 lower in households of index patients who had been vaccinated 21 days or more before testing positive than in households of unvaccinated index patients.

Vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) prevents infection and reduces the severity of coronavirus disease 2019 (Covid-19) in vaccinated persons.1,2 We investigated whether vaccination would reduce transmission in the household setting in the context of postvaccination infection. We analyzed data from the Household Transmission Evaluation Dataset (HOSTED), which has information on all laboratory-confirmed cases of Covid-19 in England and in which data on all persons sharing the same address are linked.3 We then linked to individual-level data on all Covid-19 vaccinations in England (see the Methods section in the Supplementary Appendix, available with the full text of this letter at NEJM.org). We compared the risk of secondary infection (defined as a positive SARS-CoV-2 test 2 to 14 days after the positive test for the index case) among unvaccinated household contacts of persons with SARS-CoV-2 infection who had received at least one dose of the ChAdOx1 nCoV-19 or BNT162b2 vaccine 21 days or more before testing positive with the risk among unvaccinated household contacts of unvaccinated persons with infection. We fitted logistic-regression models with adjustment for the age and sex of the person with the index case of Covid-19 (index patient) and the household contact, geographic region, calendar week of the index case, deprivation (a composite score of socioeconomic and other factors), and household type and size. We also considered the timing of effects among index patients who had been vaccinated at any time up to the date of the positive test.

Between January 4 and February 28, 2021, there were 960,765 household contacts of unvaccinated index patients, and there were 96,898 secondary cases of Covid-19 (10.1%). (Descriptive data regarding the index patients and their household contacts are provided in the Summary Results section.) The numbers of secondary cases according to the vaccination status of the index patient, and the results of logistic-regression models, are shown in Table 1. Overall, the likelihood of household transmission was approximately 40 to 50% lower in households of index patients who had been vaccinated 21 days or more before testing positive than in households of unvaccinated index patients; the findings were similar for the two vaccines. Most of the vaccinated index patients in our data set (93%) had received only the first dose of vaccine. Assessment of infection risks among household contacts according to the timing of vaccination of the index patient showed protective effects when the vaccine had been administered at least 14 days before the positive test (Figs. S1 and S2 in the Supplementary Appendix)....

Published in NEJM (June 23, 2021):

https://doi.org/10.1056/NEJMc2107717 

The spread of Covid-19 among members in a household after one person is infected is "common" and occurs quickly after illness onset, according to a new study from the US Centers for Disease Control and Prevention. The person exposed or suspected of having Covid-19 should be isolated before getting tested and before test results come back to protect others in the home, said the study, published Friday in the CDC's Morbidity and Mortality Weekly Report. "Because prompt isolation of persons with COVID-19 can reduce household transmission, persons who suspect that they might have COVID-19 should isolate, stay at home, and use a separate bedroom and bathroom if feasible," wrote a CDC-led team of researchers. In addition, all members of the household should wear masks at all times in common spaces, the team said.

Infection of others happened within days

 The research, part of an ongoing CDC-supported study, followed 101 people initially infected with Covid-19 in Nashville, Tennessee, and Marshfield, Wisconsin, between April and September.  Along with some 191 others who lived in their households, the infected people were trained to self-collect specimens -- nasal swabs only or nasal swabs and saliva samples -- each day for 14 days. In addition, each person completed a symptom diary. Infection was quick: Over half of the people (53%) who lived with someone battling Covid-19 became infected within a week, researchers found. Some 75% of these secondary infections occurred within five days of the first symptoms in the initial patient. "Substantial transmission occurred whether the index patient was an adult or a child," researchers said. A 53% household infection rate, the study said, is higher than what has been documented so far. To date, related research has reported only a 20% to 40% infection rate.  "An important finding of this study is that fewer than one half of household members with confirmed SARS-CoV-2 infections reported symptoms at the time infection was first detected," the study authors wrote. "Many reported no symptoms throughout 7 days of follow-up, underscoring the potential for transmission from asymptomatic secondary contacts and the importance of quarantine."

Isolate yourself or your loved one

 Staying isolated from other household members can be challenging, especially if you are in a small space or there are children in the home. "If you have somebody at home who's older or immunocompromised, you may want to isolate them to one side of the house so the kids and everybody else aren't around them on a regular basis," said pediatrician Dr. Tanya Altmann, editor in chief of the American Academy of Pediatrics' book "Caring for Your Baby and Young Child: Birth to Age 5 and The Wonder Years," in a prior interview.  "If it's a child that you have to care for, then you may have to make the decision to isolate one adult with the child," she added. "That adult would be caring for the child, and the other adult would be responsible for the rest of family." And of course, all of this will be extremely difficult for a single parent "who might be the only one," said pediatrician Dr. Jenny Radesky, a spokesperson for the American Academy of Pediatrics, in a prior interview. "Know who your neighbors are, even what your neighborhood social network might be," said Radesky, an assistant professor of pediatrics at the University of Michigan. "You may not be that close interpersonally, but someone may be willing to do a grocery store drop-off, or pick-up medications because we're all in this together."  The rest of the family should practice isolation as well, Radesky added. "Unfortunately you need to keep the whole household under isolation," she said. "The patterns of Covid-19 spread suggest that clusters of people who live close together are at highest risk of getting it from one another. If children are part of that household, they may show minimal symptoms but still be contagious."...

Magnitude of SARS-CoV-2 virus exposure may contribute to symptom severity. In a sample of seropositive adults (n=1101), we found that individuals who lived with a known COVID-19 case exhibited greater symptom severity and IgG concentrations compared to individuals who were seropositive but did not live with a known case (P<0.0001).

Notably, working in close proximity to others, which has received substantial attention as a risk factor for COVID-19,20 did not exhibit statistically significant associations with symptom severity or IgG antibody concentrations. Our results suggest that high virus exposure levels, which may often occur when exposed to a cohabitant with COVID-19, contribute to greater disease severity and stronger antibody responses. Regardless of which individual in the household was exposed first, our results suggest that preventing transmission within households should be a critical area of focus for public health efforts designed to reduce rates of symptomatic COVID-19. Policies and interventions that apply only to public places (e.g., mask mandates, business capacity limits) may be insufficient unless they are combined with measures that also reduce transmission within households (e.g., intensive testing, contact tracing, and isolation programs).

Preprint available in merRxiv (March 12, 2021):

https://doi.org/10.1101/2021.03.11.21253421