Wed. May 25th, 2022

A team of British scientists has recently evaluated the effectiveness of vaccines against coronavirus disease 2019 (COVID-19) in preventing household transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the UK.

Their results indicate that the vaccines effectively reduce the transmission of SARS-CoV-2 alpha and delta variants from breakthrough infections in the household environment, despite having reduced effectiveness in preventing infection. The study is currently available at medRxiv* preprint server.

Study: The effectiveness of BNT162b2 and ChAdOx1 against SARS-CoV-2 household transfer: a prospective cohort study in the UK.  Image credit: NIAID
Study: The effectiveness of BNT162b2 and ChAdOx1 against SARS-CoV-2 household transfer: a prospective cohort study in the UK. Image credit: NIAID

Background

The COVID-19 mass vaccination program was first introduced in the UK in December 2020 with the mRNA-based BNT162b2 vaccine (Pfizer / BioNTech) and adenoviral vector-based ChAdOx1 (Oxford / AstraZeneca) vaccine.

The primary purpose of the vaccination program was to protect the most vulnerable population from infection and serious illness, including elderly adults, immunocompromised patients, and those with pre-existing health conditions. In addition, healthcare professionals and other front-line staff at higher risk for SARS-CoV-2 exposure were prioritized for vaccination.

In the current study, the researchers examined the effectiveness of BNT162b2 and ChAdOx1 vaccines in preventing delta and alpha breakthrough infection and transmission among household contacts in the UK.

Study design

The study was performed on individuals with primary infections (index cases) and their respective household contacts. Information on participants’ vaccination status was obtained from the National Immunization Management System, UK.

Participants (both index cases and contacts) were asked to collect respiratory samples on days 1, 3, and 7 after recruitment using self-test kits. The samples were later tested by reverse transcription polymerase chain reaction (RT-PCR). A subset of samples were subjected to genomic sequencing to identify the alpha and delta variants.

Important observations

The survey included a total of 195 index cases and 278 contacts. On average, each household had 2 contacts. About 77% and 52% of index cases and contacts had received at least one vaccine dose at the time of recruitment, respectively. About 41% of contacts tested positive for SARS-CoV-2 within one week of recruitment.

Maximum probability of phylogeny of household index cases and contact sequences with 1,000 ultrafast bootstrap replicas anchored to the reference sequence with a 2 SNP scaled column (6.6 × 10−5 substitutions / site).  The dotted line at the bottom left indicates where a single long branch collapsed for visualization.  The non-gray shadow on the outer ring represents non-clustered households, where sequences are colored by their households.  HH002 and HH007 were the only households where none of the contacts' sequences clustered with their household index cases, and this is evidence that the contact could have had the infection elsewhere and is therefore excluded from the analysis.
Maximum probability of phylogeny of household index cases and contact sequences with 1,000 ultrafast bootstrap replicas anchored to the reference sequence with a 2 SNP scaled column (6.6 × 10−5 substitutions / site). The dotted line at the bottom left indicates where a single long branch collapsed for visualization. The non-gray shadow on the outer ring represents non-clustered households, where sequences are colored by their households. HH002 and HH007 were the only households where none of the contacts’ sequences clustered with their household index cases, and this is evidence that the contact could have had the infection elsewhere and is therefore excluded from the analysis.

Age- and variant-specific risk of infection

Of all index cases, 99 had the alpha infection, 24 had delta infection and 52 had infections with unknown variants. In addition, 20 index cases did not test positive again after recruitment.

In the absence of vaccination of either cases or contacts, the risk of delta infection transmission was 1.64 times higher than the risk of alpha infection transmission. Compared to children, adult contacts over the age of 18 were 1.19 times more susceptible to delta infection. The risk of viral transmission from delta index cases aged 50 years or older was relatively higher than those aged 18-49 years.

Predicted secondary rates of attack (SARs) for each combination of case and contact vaccine status.  Large numbers inside cells are the median of SAR, where the small numbers below and above correspond to the credible range of 95%.
Predicted secondary rates of attack (SARs) for each case and contact vaccine status combination. Large numbers inside cells are the median of SAR, where the small numbers below and above correspond to the credible range of 95%.

Vaccine efficacy

The efficacy of two doses of BNT162b2 and ChAdOx1 vaccines against delta infection transmission was 31% and 42%, respectively. The efficacy was similar for alpha infection transmission.

In terms of protection against acquiring alpha and delta infections, the BNT162b2 vaccine had 71% and 24% efficacy, respectively. For the ChAdOx1 vaccine, efficacy was 26% and 14% against alpha and delta infections, respectively.

Incidence of secondary infection

In unvaccinated households, the incidence of secondary infections with alpha and delta variants among adults was 49% and 81%, respectively. The highest efficacy of both vaccines against the secondary infection rate was observed when both cases and contacts were fully vaccinated.

The significance of the study

The study results reveal that both mRNA-based and adenovirus vaccine vector COVID-19 vaccines effectively reduce the risk of viral transmission from individuals who have developed alpha or delta infection despite being fully vaccinated. However, both vaccines have relatively lower efficacy in preventing delta infections.

The significant level of protection against SARS-CoV-2 transmission in households observed in the study highlights the importance of COVID-19 vaccines to limit the risk of COVID-19, especially in indoor settings where cases and contacts remain close on each other for an extended period of time.

*Important message

medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered essential, guide clinical practice / health-related behavior or be treated as established information.[if–>

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