Myth: Vaccines Don’t Stop Transmission

I don’t know why people make this claim. Does transmission still occur after vaccination? Of course, but it still reduces the risk of transmission. It seems that those making this claim are using simplistic, binary thinking. One obvious reason that the likelihood of transmission is reduced is that the vaccines reduce viral load. This is part of the dose-response dynamic in disease spread. My commentary is denoted by brackets [Note: ], everything else is direct quotes.

Infectiousness of SARS-CoV-2 breakthrough infections and reinfections during the Omicron wave (2023)

We estimated that index cases who had received ≥1 COVID-19 vaccine doses had a 22% (6–36%) lower risk of transmitting infection than unvaccinated index cases. In analyses that further accounted for the number of vaccine doses received by an index case, each additional dose was associated with an average 11% reduction (5–17%) in risk of transmission to the close contact.

A key result is that the vaccine-mediated reduction in infectiousness of SARS-CoV-2 breakthrough infections appears to be dose dependent. Each dose of the vaccine provided an additional average 11% relative reduction in infectiousness, which was mostly driven by residents with a booster dose. The findings of this study support the indirect effects of COVID-19 vaccination (especially booster doses) to slow transmission of SARS-CoV-2 and build on evidence of the direct effects of COVID-19 vaccination23 to emphasize the overall importance of COVID-19 vaccination.

Association of mRNA Vaccination With Clinical and Virologic Features of COVID-19 Among US Essential and Frontline Worker (2022)

Participants with Delta and Omicron infections who received the second vaccine dose 14 to 149 days before infection had a significantly reduced mean viral load compared with unvaccinated participants (3 vs 4.1 log10 copies/μL; mean difference, −1.0 [95% CI, −1.7 to −0.2] log10 copies/μL for Delta; 2.8 vs 3.5 log10 copies/μL; mean difference, −1.0 [95% CI, −1.7 to −0.3] log10 copies/μL for Omicron)

Mean PFU was significantly lower among participants who received dose 2 at least 150 days before breakthrough infection or the third dose 7 to 149 days before breakthrough infection compared with unvaccinated participants (4.1 vs 4.8; mean difference, −0.8 [95% CI, −1.5 to −0.1] PFU/mL and 3.1 vs 4.8; mean difference, −1.6 [95% CI, −2.8 to −0.5] PFU/mL). [Note: PFU is plague-forming units.]

Although viral RNA shedding cannot be directly attributable to transmission, the relatively high viral load of Omicron infections together with the higher frequency of asymptomatic infection supports previous studies suggesting an association with increased transmission, particularly during the first 3 to 5 days when viral load peaked.

Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study (2022)

Interpretation: Vaccination reduces the risk of delta variant infection and accelerates viral clearance.

Infectious viral load in unvaccinated and vaccinated individuals infected with ancestral, Delta or Omicron SARS-CoV-2 (2002)

There is extensive evidence that vaccines against SARS-CoV-2, which target the original strain, reduce infection case numbers and disease severity.

[Note: DPOS is days post onset of symptoms. The important parts of this graph are the red and black lines. the shaded area around them is the 95% CI. At day three, it becomes apparent that there is about a 10-fold reduction in infectious viral load among those who were vaccinated. This is pre-delta strains.]

[Note: The above is the same comparison as the prior graph, but this is with delta. One important difference is that the reduction in infectious viral load was present even at the start of the onset of symptoms. I would argue that this reduction is also present before the onset of symptoms, suggesting a reduction in the risk of asymptomatic transmission.]

[Note: The same reduction pattern is seen for omicron (above) as well.]

In conclusion, this study provides significant evidence for higher infectiousness of SARS-CoV-2 Delta as well as a significant effect of full vaccination on infectious VL and its speed of clearance. In addition, we show that Omicron BA.1 has lower infectious VLs compared to Delta in fully vaccinated individuals. Last, after Omicron BA.1 infection, lower infectious VL is observed only in boosted individuals. Our findings highlight the beneficial effect of vaccinations beyond the individual protection from severe disease and underscore the importance of booster vaccination.

Household transmission of the SARS-CoV-2 Omicron variant in Denmark (2022)

For households infected with the Delta VOC, we estimated an OR of infection of 2.36 (95% confidence interval, CI: 2.20–2.54) for unvaccinated contacts compared to fully vaccinated contacts, and an OR of 0.41 (CI: 0.36–0.47) for booster-vaccinated contacts compared to fully vaccinated contacts, after adjustment for confounders (age and sex of the primary case, age and sex of the contact, and household size). The corresponding OR estimates for households infected with the Omicron VOC was 1.09 (CI: 0.99–1.20) for unvaccinated contacts and 0.55 (CI: 0.48–0.63) for booster-vaccinated contacts, both compared to fully vaccinated contacts. With no interaction between vaccine status and variant, unvaccinated primary cases were associated with an OR of infection of 1.37 (CI: 1.27–1.47) compared to fully vaccinated primary cases, while booster-vaccinated primary cases were associated with a decreased OR of infection of 0.80 (CI: 0.69–0.92). This demonstrates a baseline association between vaccination status and both susceptibility and infectiousness.

Transmission and Infectious SARS-CoV-2 Shedding Kinetics in Vaccinated and Unvaccinated Individuals (2022)

In this cohort study, although the initial genomic viral load was similar between vaccinated and unvaccinated individuals, fully vaccinated individuals had a shorter duration of viable viral shedding and a lower secondary attack rate than partially vaccinated or unvaccinated individuals. Data from this study provide important evidence that despite the possibility of breakthrough infections, COVID-19 vaccinations remain critically useful for controlling the spread of SARS-CoV-2.

Vaccination with BNT162b2 reduces transmission of SARS-CoV-2 to household contacts in Israel (2022)

Vaccination reduced susceptibility to infection by 89.4% [95% confidence interval (CI): 88.7 to 90.0%], whereas vaccine effectiveness against infectiousness given infection was 23.0% (95% CI: −11.3 to 46.7%) during days 10 to 90 after the second dose, before 1 June 2021. Total vaccine effectiveness was 91.8% (95% CI: 88.1 to 94.3%). However, vaccine effectiveness is reduced over time as a result of the combined effect of waning of immunity and emergence of the Delta variant.

Longitudinal Analysis of SARS-CoV-2 Vaccine Breakthrough Infections Reveals Limited Infectious Virus Shedding and Restricted Tissue Distribution (2022)

The durations of both infectious virus shedding and symptoms were significantly reduced in vaccinated individuals compared with unvaccinated individuals. We also observed that breakthrough infections are associated with strong tissue compartmentalization and are only detectable in saliva in some cases.

Indirect protection of children from SARS-CoV-2 infection through parental vaccination (2022)

We found that having a single vaccinated parent was associated with a 26.0 and a 20.8% decreased risk in the early and late periods, respectively, and having two vaccinated parents was associated with a 71.7 and a 58.1% decreased risk, respectively. Thus, parental vaccination confers substantial protection on unvaccinated children in the household.

Impact of population mixing between vaccinated and unvaccinated subpopulations on infectious disease dynamics: implications for SARS-CoV-2 transmission (2022)

Results: We found that the risk of infection was markedly higher among unvaccinated people than among vaccinated people under all mixing assumptions. The contact-adjusted contribution of unvaccinated people to infection risk was disproportionate, with unvaccinated people contributing to infections among those who were vaccinated at a rate higher than would have been expected based on contact numbers alone. We found that as like-with-like mixing increased, attack rates among vaccinated people decreased from 15% to 10% (and increased from 62% to 79% among unvaccinated people), but the contact-adjusted contribution to risk among vaccinated people derived from contact with unvaccinated people increased.

Interpretation: Although risk associated with avoiding vaccination during a virulent pandemic accrues chiefly to people who are unvaccinated, their choices affect risk of viral infection among those who are vaccinated in a manner that is disproportionate to the portion of unvaccinated people in the population.

Conclusion: Using simple mathematical modelling, we have shown that, although risk associated with avoiding vaccination during a virulent pandemic accrues chiefly to those who are unvaccinated, the choice of some individuals to refuse vaccination is likely to affect the health and safety of vaccinated people in a manner disproportionate to the fraction of unvaccinated people in the population.

Effect of Vaccination on Transmission of SARS-CoV-2 (2021)

Cases of Covid-19 were less common among household members of vaccinated health care workers during the period beginning 14 days after the first dose than during the unvaccinated period before the first dose (event rate per 100 person-years, 9.40 before the first dose and 5.93 beginning 14 days after the first dose). After the health care worker’s second dose, the rate in household members was lower still (2.98 cases per 100 person-years). These differences persisted after fitting extended Cox models that were adjusted for calendar time, geographic region, age, sex, occupational and socioeconomic factors, and underlying conditions. Relative to the period before each health care worker was vaccinated, the hazard ratio for a household member to become infected was 0.70 (95% confidence interval [CI], 0.63 to 0.78) for the period beginning 14 days after the first dose and 0.46 (95% CI, 0.30 to 0.70) for the period beginning 14 days after the second dose.

Vaccine effectiveness against SARS-CoV-2 transmission to household contacts during dominance of Delta variant (B.1.617.2), the Netherlands, August to September 2021 (2021)

Our results indicate that vaccination confers protection against onward transmission of SARS-CoV-2 from vaccinated index cases, albeit somewhat less for the Delta than for the Alpha variant. The VET to unvaccinated household contacts is higher than to vaccinated household contacts, with the latter already largely protected from infection and especially from severe disease by their own vaccine-induced immunity.