Table 3.

Effect of Vaccination on Peak Respiratory Tract Viral Load In Breakthrough Cases

Study Type / Size	Setting	Effect Size
RCT / 8534 (520 infections) [27]	UK	Infected participants who were vaccinated had median minimum cycle threshold of 28.8 (IQR 20.5–33.5) vs. 20.2 (IQR 15.5–29.6) in the unvaccinated group (p < 0.0001); duration of PCR positivity 1 week shorter
Observational / 3975 (204 infections) [13]	The United States	Infected participants who were partially or fully vaccinated had a 40% reduction (95% CI 16–57%) in viral load relative to unvaccinated participants, and viral RNA could be detected for 6.2 fewer days (95% CI 4.0–8.4)
Observational / 10 infections [37]	US nursing home	2.4 mean log10 lower viral load at diagnosis after first dose compared with unvaccinated
Observational / 373 402 (12525 infections) [14]	UK	88% reduction in infection with Ct < 30 (95% CI 80–93%, p < 0.001) relative to Ct ≥30 after 2 vaccine doses
Observational / 1888 post vaccination infections [38]	Israel	2.8–4.5 fold reduction in viral loads for post vaccination infections 12–37 days after first dose compared with matched unvaccinated controls
Observational / 16 297 positive tests [39]	Israel	Viral loads were 1.6 to 20 fold lower among individuals >60 (75% vaccinated) compared with those age 40–60 (<25% vaccinated)