Transmission

"Vaccination will take us far enough along, not that we're out of the woods, but that we're really out of the fire," University of Toronto epidemiologist Colin Furness said.

Breakthrough COVID-19 cases in vaccinated people may be less likely to spread infection because virus is shed for a shorter period of time as opposed to infections in unvaccinated people.

Antivaxxers are touting a "revelation" that wasn't that the phase 3 clinical trial of the Pfizer COVID vaccine didn't examine its ability to block transmission.

The COVID-19 vaccine clinical trials were designed to study the vaccine’s safety and efficacy in preventing symptomatic disease, not transmission. But online publications now misleadingly present the fact that the Pfizer/BioNTech vaccine was not tested for transmission as a “shocking admission” and proof that the company and the government lied.

Evidence is increasing that, not only do COVID-19 vaccines either stop you getting sick or substantially reduce the severity of your symptoms, they’re also likely to substantially reduce the chance of transmitting the virus to others.

Lessons from other viruses show that even if vaccines don’t completely stop disease spread, they can still successfully contain it

Even as highly contagious coronavirus variants are circulating, experts say leading vaccines are not only preventing infections but likely curbing transmission as well. But that doesn't mean the vaccinated can fully protect the unvaccinated, particularly in areas with lower vaccination rates.

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Background Pre-Delta, vaccination reduced SARS-CoV-2 transmission from individuals infected despite vaccination, potentially via reducing viral loads. While vaccination still lowers the risk of infection, similar viral loads in vaccinated and unvaccinated individuals infected with Delta question how much vaccination prevents transmission. Methods We performed a retrospective observational cohort study of adult contacts of SARS-CoV-2-infected adult index cases using English contact testing data. We used multivariable Poisson regression to investigate associations between transmission and index case and contact vaccination, and how these vary with Alpha and Delta variants (classified using S-gene detection/calendar trends) and time since second vaccination. Results 54,667/146,243(37.4%) PCR-tested contacts of 108,498 index cases were PCR-positive. Two doses of BNT162b2 or ChAdOx1 vaccines in Alpha index cases were independently associated with reduced PCR-positivity in contacts (aRR, adjusted rate ratio vs. unvaccinated=0.32[95%CI 0.21-0.48] and 0.48[0.30-0.78] respectively). The Delta variant attenuated vaccine-associated reductions in transmission: two BNT162b2 doses reduced Delta transmission (aRR=0.50[0.39-0.65]), more than ChAdOx1 (aRR=0.76[0.70-0.82]). Variation in Ct values (indicative of viral load) explained 7-23% of vaccine-associated transmission reductions. Transmission reductions declined over time post-second vaccination, for Delta reaching similar levels to unvaccinated individuals by 12 weeks for ChAdOx1 and attenuating substantially for BNT162b2. Protection in contacts also declined in the 3 months post-second vaccination. Conclusions Vaccination reduces transmission of Delta, but by less than the Alpha variant. The impact of vaccination decreased over time. Factors other than PCR Ct values at diagnosis are important in understanding vaccine-associated transmission reductions. Booster vaccinations may help control transmission together with preventing infections. ### Competing Interest Statement DWE declares lecture fees from Gilead outside the submitted work. No other author has a conflict of interest to declare. ### Funding Statement This study was funded by the UK Government's Department of Health and Social Care. This work was supported by the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Healthcare Associated Infections and Antimicrobial Resistance at Oxford University in partnership with Public Health England (PHE) (NIHR200915), and the NIHR Biomedical Research Centre, Oxford. The views expressed in this publication are those of the authors and not necessarily those of the NHS, the National Institute for Health Research, the Department of Health or Public Health England. DWE is a Robertson Foundation Fellow and an NIHR Oxford BRC Senior Fellow. ASW is an NIHR Senior Investigator. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study was performed as public health surveillance and NHS Test and Trace program quality assurance, under Section 251 of the NHS Act 2006 with approvals from Public Health England (PHE), the Department of Health and Social Care and NHS Test and Trace. PHE's Research Ethics and Governance Group (PHE's Research Ethics Committee) reviewed the study protocol and confirmed compliance with all regulatory requirements. As no regulatory or ethical issues were identified, it was agreed that full ethical review was not needed, and the protocol was approved. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes Applications to use the data in this study can be made to NHS Digital's Data Access Request Service, please see for more details.

The World Health Organization has walked back a top official’s claim that asymptomatic people rarely spread the coronavirus to others. The WHO’s confused messaging on the topic drew criticism from public health officials. The Harvard Global Health Institute said, “In fact, some evidence suggests that people may be most infectious in the days before they become symptomatic — that is, in the presymptomatic phase when they feel well, have no symptoms, but may be shedding substantial amounts of virus.” Meanwhile, two new papers published in the journal Nature find coronavirus lockdowns saved millions of lives and averted tens of millions of infections worldwide. UC Berkeley researcher Solomon Hsiang co-authored one of the reports. Solomon Hsiang: “Never in human history have so many people around the world come together, coordinated their actions and worked to save so many lives in such a short period of time.”

Five years later, the greatest basic science failure in generations caused the pandemic harms highlighted by people across the political spectrum, and broke our social cohesion.

Q: How do people who have not been vaccinated against COVID-19 pose a risk to people who have been vaccinated? A: An unvaccinated person who is infected with COVID-19 poses a much greater risk to others who are also unvaccinated. But vaccines are not 100% effective, so there is a chance that an unvaccinated person could infect a vaccinated person — particularly the vulnerable, such as elderly and immunocompromised individuals.