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 

A slew of detailed studies has now quantified the increased risk the virus poses to older people, men, and other groups. For every 1,000 people infected with the coronavirus who are under the age of 50, almost none will die. For people in their fifties and early sixties, about five will die — more men than women. The risk then climbs steeply as the years accrue. For every 1,000 people in their mid-seventies or older who are infected, around 116 will die. These are the stark statistics obtained by some of the first detailed studies into the mortality risk for COVID-19. Trends in coronavirus deaths by age have been clear since early in the pandemic. Research teams looking at the presence of antibodies against SARS-CoV-2 in people in the general population — in Spain, England, Italy and Geneva in Switzerland — have now quantified that risk, says Marm Kilpatrick, an infectious-disease researcher at the University of California, Santa Cruz. “It gives us a much sharper tool when asking what the impact might be on a certain population that has a certain demographic,” says Kilpatrick. The studies reveal that age is by far the strongest predictor of an infected person’s risk of dying — a metric known as the infection fatality ratio (IFR), which is the proportion of people infected with the virus, including those who didn’t get tested or show symptoms, who will die as a result. “COVID-19 is not just hazardous for elderly people, it is extremely dangerous for people in their mid-fifties, sixties and seventies,” says Andrew Levin, an economist at Dartmouth College in Hanover, New Hampshire, who has estimated that getting COVID-19 is more than 50 times more likely to be fatal for a 60-year-old than is driving a car.

But “age cannot explain everything”, says Henrik Salje, an infectious-disease epidemiologist at the University of Cambridge, UK. Gender is also a strong risk factor, with men almost twice more likely to die from the coronavirus than women. And differences between countries in the fatality estimates for older age groups suggest that the risk of dying from coronavirus is also linked to underlying health conditions, the capacity of health-care systems, and whether the virus has spread among people living in elderly-care facilities.

To estimate the mortality risk by age, researchers used data from antibody-prevalence studies. In June and July, thousands of people across England received a pinprick antibody test in the post. Of the 109,000 randomly selected teenagers and adults who took the test, some 6% harboured antibodies against SARS-CoV-2. This result was used to calculate an overall IFR for England of 0.9% — or 9 deaths in every 1,000 cases. The IFR was close to zero for people between the ages of 15 and 44, increasing to 3.1% for 65–74-year-olds and to 11.6% for anyone older. The results of the study have been posted to the medRxiv preprint server.

Another study from Spain that started in April, and tested for antibodies in more than 61,000 residents in randomly selected households, observed a similar trend. The overall IFR for the population was about 0.8%, but it remained close to zero for people under 50, before rising swiftly to 11.6% for men 80 years old and over; it was 4.6% for women in that age group. The results also revealed that men are more likely to die of the infection than are women — the gap increasing with age. “Men face twice the risk of women,” says Beatriz Pérez-Gómez, an epidemiologist at the Carlos III Institute of Health in Madrid, who was involved in the Spanish study. The results have also been posted to the medRxiv server.Differences in the male and female immune-system response could explain the divergent risks, says Jessica Metcalf, a demographer at Princeton University, New Jersey. “The female immune system might have an edge by detecting pathogens just a bit earlier,” she says...