Transmissibility of COVID-19 among Vaccinated Individuals: A Rapid Literature Review - Update #1

  1. Oluwaseun Egunsola
  2. Liza Mastikhina
  3. Laura Dowsett
  4. Brenlea Farkas
  5. Mark Hofmeister
  6. Lynora Saxinger
  7. Fiona Clement

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Abstract

Objectives

This is an update of a previous report that examined literature published up to March 11th, 2021. Sixteen additional studies have been included in this update. The objective of this report is to identify comparative observational studies and randomized controlled trials (RCTs) evaluating the efficacy and effectiveness of COVID-19 vaccination in reducing forward transmission from vaccinated people, and studies examining the biological plausibility of vaccination-induced transmission reduction.

Method

A search of databases, MEDLINE, Embase, L-OVE and the Cochrane Central Register of Controlled Trials was conducted to identify RCTs or comparative observational studies evaluating the efficacy and effectiveness of COVID-19 vaccination in the prevention of transmission, asymptomatic infections and transmissibility of COVID-19 among vaccinated persons. An additional search of grey literature was conducted. This search is current to May 4th, 2021.

Results

In this update, 16 additional studies, including 9 human and 7 animal studies, were included. Therefore, this review examines a total of 33 included studies: 21 human studies and 12 preclinical animal studies. Evidence from two large household surveillance studies from the UK suggests that a single or full dose of AstraZeneca (AZ) and Pfizer-BioNtech (PfBnT) vaccines may prevent household transmission of COVID-19 after 14 days of vaccination by up to 54%. The AZ vaccine trials in the general population suggest that an initial low dose followed by a standard dose may provide up to 59% protection against asymptomatic or unknown infection, although efficacy against these outcomes was not demonstrated following two standard doses. PfBnT vaccine observational studies in the general population suggest up to 90% effectiveness against asymptomatic infection after seven or more days of full dose vaccination. Up to 75% effectiveness against asymptomatic infection was reported after full- dose in healthcare workers. Across RCTs examining asymptomatic infection in the general population, one dose of Moderna was shown to provide an efficacy of 61.4% against asymptomatic infection 21 days after the first dose; in another trial, the J&J vaccine had an efficacy of 74% 28 days after the first dose. Lastly, seven of eight studies found significantly increased cycle threshold, suggestive of lower viral load, in PfBnT or AZ vaccinated individuals compared with those who were unvaccinated.

Conclusion

The AZ and PfBnT vaccines may prevent household transmission of COVID-19 after 14 days of vaccination. More studies have found the vaccines to significantly reduce the risk of asymptomatic infection and significantly increase cycle threshold, suggestive of lower viral load. Further research is needed to evaluate post-vaccination infectivity and transmission of both the wild type COVID-19 virus and the variants of concern from other jurisdictions.

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  1. ScreenIT

    SciScore for 10.1101/2021.06.29.21255526: (What is this?)

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

    Table 1: Rigor

    EthicsIACUC: Quality assessment was conducted based on study design: Cochrane risk of bias for non- randomized studies (ROBINS-I) for non-randomized studies,33 Cochrane Risk of Bias (version 5.1.0) for human-subject RCTs,34 and the Systematic Review Centre for Laboratory animal Experimentation’s (SYRCLE) risk of bias for animal studies.
    Sex as a biological variablenot detected.
    RandomizationQuality assessment was conducted based on study design: Cochrane risk of bias for non- randomized studies (ROBINS-I) for non-randomized studies,33 Cochrane Risk of Bias (version 5.1.0) for human-subject RCTs,34 and the Systematic Review Centre for Laboratory animal Experimentation’s (SYRCLE) risk of bias for animal studies.
    Blindingnot detected.
    Power Analysisnot detected.

    Table 2: Resources

    Software and Algorithms
    SentencesResources
    The MEDLINE strategy was peer reviewed by another senior information specialist prior to execution using the PRESS Checklist.
    MEDLINE
    suggested: (MEDLINE, RRID:SCR_002185)
    32 Using the multifile option and deduplication tool available on the OVID platform, we searched Ovid MEDLINE®, including Epub Ahead of Print, In-Process & Other Non-Indexed Citations, Embase, and EBM Reviews - Cochrane Central Register of Controlled Trials.
    Embase
    suggested: (EMBASE, RRID:SCR_001650)
    Results were downloaded and deduplicated using EndNote version 9.3.3 (Clarivate Analytics) and uploaded to word.
    EndNote
    suggested: (EndNote, RRID:SCR_014001)
    Quality assessment was conducted based on study design: Cochrane risk of bias for non- randomized studies (ROBINS-I) for non-randomized studies,33 Cochrane Risk of Bias (version 5.1.0) for human-subject RCTs,34 and the Systematic Review Centre for Laboratory animal Experimentation’s (SYRCLE) risk of bias for animal studies.
    Cochrane Risk of Bias
    suggested: None

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    There were significant limitations to many of the included studies. The primary endpoint of the vaccine randomized controlled trials were detection of test positive symptomatic COVID-19, however some studies presented data, which suggest a reduction in the likelihood of testing positive for SARS-CoV-2 RT-PCR in the absence of documented symptoms after vaccination. Furthermore, it was not possible to directly compare findings across studies owing to variations in the assessment of symptom status, and the testing used and timing of these assessments. Also, the possibility of persistent PCR positivity after COVID-19 infection51 could not be excluded in some of the studies without baseline PCR assessment. Few studies included surveillance nasal swabs for PCR positivity. Most of the current data were around viral detection, rather than evidence of cultivatable virus. Therefore, there was limited data to evaluate the efficacy or effectiveness of COVID-19 vaccines in decreasing viral loads. In addition, there are only a limited number of epidemiologic data addressing evidence of forward transmission after vaccination. Based on the current evidence, we suggest the following:

    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.

    About SciScore

    SciScore is an automated tool that is designed to assist expert reviewers by finding and presenting formulaic information scattered throughout a paper in a standard, easy to digest format. SciScore checks for the presence and correctness of RRIDs (research resource identifiers), and for rigor criteria such as sex and investigator blinding. For details on the theoretical underpinning of rigor criteria and the tools shown here, including references cited, please follow this link.

  2. Version published to 10.1101/2021.06.29.21255526 on medRxiv