Effectiveness of BNT162b2 and ChAdOx1 against SARS-CoV-2 household transmission: a prospective cohort study in England

  1. Samuel Clifford
  2. Pauline Waight
  3. Jada Hackman
  4. Stephane Hué
  5. Charlotte M Gower
  6. Freja CM Kirsebom
  7. Catriona Skarnes
  8. Louise Letley
  9. Jamie Lopez Bernal
  10. Nick Andrews
  11. Stefan Flasche
  12. Elizabeth Miller

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Abstract

Background

The ability of SARS-CoV-2 vaccines to protect against infection and onward transmission determines whether immunisation can control global circulation. We estimated effectiveness of BNT162b2 and ChAdOx1 vaccines against acquisition and transmission of the Alpha and Delta variants in a prospective household study in England.

Methods

Adult index cases in the community and their household contacts took oral-nasal swabs on days 1, 3 and 7 after enrolment. Swabs were tested by RT-qPCR with genomic sequencing conducted on a subset. We used Bayesian logistic regression to infer vaccine effectiveness against acquisition and transmission, adjusted for age, vaccination history and variant.

Findings

Between 2 February 2021 and 10 September 2021 213 index cases and 312 contacts were followed up. After excluding households lacking genomic proximity (N=2) or with unlikely serial intervals (N=16), 195 households with 278 contacts remained of whom 113 (41%) became PCR positive. Delta lineages had 1.64 times the risk (95% Credible Interval: 1.15 – 2.44) of transmission than Alpha; contacts older than 18 years were 1.19 times (1.04 - 1.52) more likely to acquire infection than children. Effectiveness of two doses of BNT162b2 against transmission of Delta was 31% (−3%, 61%) and 42% (14%, 69%) for ChAdOx1, similar to their effectiveness for Alpha. Protection against infection with Alpha was higher than for Delta, 71% (12%,95%) vs 24% (−2%, 64%) respectively for BNT162b2 and 26% (−39%, 73%) vs 14% (−5%, 46%) respectively for ChAdOx1.

Interpretation

BNT162b2 and ChAdOx1 reduce transmission of the Delta variant from breakthrough infections in the household setting though their protection against infection is low.

Funding

This study was funded by the UK Health Security Agency (formerly Public Health England) as part of the COVID-19 response.

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  1. ScreenIT

    SciScore for 10.1101/2021.11.24.21266401: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Ethicsnot detected.
    Sex as a biological variablenot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.

    Table 2: Resources

    No key resources detected.

    Results from OddPub: Thank you for sharing your code and data.

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    24 Our study comes with limitations, most importantly the potential for misclassification of the direction of transmission, the lack of inclusion of waning vaccine protection and the diversity of vaccines lineages and age groups in the dataset. To minimise the potential for misclassification we restrict the main analyses to only those putative transmission pairs where the there was no evidence against direct transmission based on phylogenetic distance (which was available for 63% of all putative transmission pairs) and where symptom onset in the contact did not pre-date that of the index case by more than 2 days. If residual misclassification between infector and infected remained this would re-attribute infection protection to transmission protection and vice versa. We also did not include waning of vaccine protection in our analyses26In the analysed dataset the longest reported time since vaccine receipt was 169 days. While some individuals in the analysis have since become eligible for booster vaccination over concerns of waning protection some of this potential effect will have been absorbed in our model in the age structuring because of the strong correlation between age and timing of vaccine eligibility as per vaccine roll-out strategy in the UK. Lastly, data collection spanned a period of multiple months during which Delta became the dominant strain in circulation in the UK and included participants vaccinated with two different vaccine products; thus requiring sub-str...

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    Results from scite Reference Check: We found no unreliable references.

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  2. Version published to 10.1101/2021.11.24.21266401v2 on medRxiv
  3. Version published to 10.1101/2021.11.24.21266401v1 on medRxiv