COVID vaccines effectively reduce outbreaks of SARS-CoV-2 alpha and delta virus in households

A team of UK-based scientists recently evaluated the efficacy of 2019 coronavirus disease (COVID-19) vaccines in preventing home transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 ) in England.

Their results indicate that the vaccines effectively reduce the transmission of alpha and delta variants of SARS-CoV-2 from breakthrough infections in the home setting, despite reduced efficacy in preventing infections. The study is currently available on the medRxiv* preprint server.

Study: Effectiveness of BNT162b2 and ChAdOx1 against domestic transmission of SARS-CoV-2: a prospective cohort study in England. Image Credit: NIAID

Background

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

The main objective of the vaccination program was to protect the most vulnerable population against serious infections and diseases, including the elderly, immunocompromised patients and those with pre-existing health problems. Additionally, healthcare workers and other front-line workers who are at higher risk of exposure to SARS-CoV-2 have been prioritized for immunization.

In the present study, scientists investigated the effectiveness of the BNT162b2 and ChAdOx1 vaccines in preventing infections and delta and alpha transmission among household contacts in England.

Study design

The study was carried out on people with primary infections (index cases) and their respective family contacts. Information on the immunization status of participants 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. Samples were later tested by reverse transcription polymerase chain reaction (RT-PCR). A subset of samples were subjected to genomic sequencing to identify alpha and delta variants.

Important Notes

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

Maximum likelihood phylogeny of domestic index cases and contact sequences with 1000 ultra-fast bootstrap replicates rooted in the reference sequence with a graduated bar of 2 SNPs (6.6 × 10−5 substitutions / site).  The dotted line at the bottom left indicates where a single long branch has been cut for viewing.  The non-gray shading on the outer ring represents ungrouped households where the sequences are colored by their households.  HH002 and HH007 were the only households where none of the contact sequences clustered with their household's index case and this shows that the contact might have contracted the infection elsewhere and is therefore excluded from the analysis.
Maximum likelihood phylogeny of domestic index cases and contact sequences with 1000 ultra-fast bootstrap replicates rooted in the reference sequence with a graduated bar of 2 SNPs (6.6 × 10−5 substitutions / site). The dotted line at the bottom left indicates where a single long branch has been cut for viewing. The non-gray shading on the outer ring represents ungrouped households where the sequences are colored by their households. HH002 and HH007 were the only households where none of the contact sequences clustered with their household’s index case and this shows that the contact might have contracted the infection elsewhere and is therefore excluded from the analysis.

Risk of infection specific to age and variant

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

In the absence of vaccination of cases or contacts, the risk of transmitting delta infection was 1.64 times greater than that of transmitting alpha infection. Compared with children, adult contacts over the age of 18 were 1.19 times more susceptible to delta infection. The risk of viral transmission of index delta cases aged 50 or older was comparatively higher than those aged 18 to 49.

Predicted Secondary Attack Rate (SAD) for each combination of case and contact immunization status.  Large numbers inside cells are the median SAR, small numbers below and above are the 95% credible range.
Predicted Secondary Attack Rate (SAD) for each combination of case and contact immunization status. Large numbers inside cells are the median SAR, small numbers below and above are the 95% credible range.

Vaccine efficacy

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

Regarding protection against the acquisition of alpha and delta infections, the BNT162b2 vaccine had an efficacy of 71% and 24%, respectively. For the ChAdOx1 vaccine, the efficacy was 26% and 14% against alpha and delta infections, respectively.

Prevalence of secondary infection

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

Importance of the study

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

The considerable level of protection against domestic transmission of SARS-CoV-2 observed in the study underscores the importance of COVID-19 vaccines in limiting the risk of COVID-19, especially in indoor facilities where cases and contacts stay nearby for an extended period of time.

*Important Notice

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

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