The secondary household transmission rate is lower in vaccinated individuals who were infected with SARS-CoV-2 when compared to individuals who were infected but had not taken the vaccine, according to a new study from the UK published in the New England Journal of Medicine.

The objective of the study was to investigate if Covid-19 vaccine would reduce transmission in the household setting with post-vaccination infection. For this, the researchers utilized the Household Transmission Evaluation Dataset (HOSTED) data, which has information on all laboratory-confirmed cases of Covid-19 in England. It also links data on all persons residing at the same address. This data was linked to data from the National Immunisation Management System (NIMS) to obtain information such as the dates and types of Covid-19 vaccinations for all individuals vaccinated in England. The analysis included households with an index case occurring from 4th January 2021 up to 28th February 2021, with 14 days observable follow up for all contacts.

The risk of secondary infection among unvaccinated household contacts of persons with SARS-CoV-2 infection who had received at least one dose of the AstraZeneca-Oxford vaccine or the Pfizer-BioNTech vaccine ≥21 days before testing positive was compared with the risk of secondary infection among unvaccinated household contacts of unvaccinated persons with infection.

• Index case was defined as the earliest case of laboratory-confirmed COVID-19, by diagnosis date, for a household.
• Household contacts were defined as all persons with the same address as the index cases.
• Secondary cases were defined as a known household contact of an index case with a positive SARS-CoV-2 test that has a specimen date 2-14 days after the specimen date of the index case.

The odds of being a secondary case was modelled using logistic regression and adjusted for the age and sex of the index case 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.

In homes where the index case was not vaccinated before testing positive, there were 96,898 secondary cases out of 960,765 household contacts indicating an overall secondary attack rate of 10.1% in households with unvaccinated index cases.  

• Among the households where the index patient had received the AstraZeneca vaccine more than 21 days before testing positive, 196 secondary cases (secondary attack rate of 5.7%) were detected out of 3424 contacts.
• In households where the index patient had received the Pfizer vaccine more than 21 days before testing positive, 371 (secondary attack rate of 6.25%) secondary cases were detected out of 5939 contacts.
• Overall, vaccination with either the Pfizer or AstraZeneca vaccine ≥21 days before testing positive reduced the probability of household transmission by 40-50% vs households of unvaccinated index patients. Analysis of data showed that 93% of the vaccinated index patients had taken only the first dose of vaccine.

These results indicate that the risk of household transmission is reduced if the index patient had taken the vaccine ≥14 days before testing positive. But this protective effect was seen to sharply decline when the vaccine was taken closer to the positive test date.

Covid-19 vaccines have been shown to reduce severity of the disease. The efficacy of different vaccines ranges from 50% (Sinovac) to more than 65% (J&J) to more than 70% (Covaxin, AstraZeneca/Covishield) to more than 90% (Pfizer/BioNTech, Moderna, Sputnik V). By decreasing the number of people with Covid-19 as well as viral load in those who do develop the infection, vaccines may also significantly reduce transmission (GAVI). This will eventually reduce the burden on the overwhelmed healthcare system.

Aggressive vaccination is the need of the hour. The faster we vaccinate people, the sooner we are likely to see a world free of the pandemic. Despite reports of adverse effects, the balance certainly tilts in favour of the vaccines. Everyone eligible must take the vaccine.

(Source: NEJM, June 23, 2021, DOI: 10.1056/NEJMc2107717)