Do Covid vaccines prevent transmission?

The short version: the evidence that COVID-19 vaccines prevent onward transmission is thin, wanes fast, and was never tested against a true unvaccinated control for the question that matters.

The best study in this set comes from Israel, where researchers measured viral load (Ct values) in over 300,000 people during the Delta and Omicron waves [16]. A higher Ct value means less virus, which should mean less transmission. During Delta, a recent third dose raised Ct by about 1.5 units compared to the unvaccinated. During Omicron, that effect was much smaller and faded to near zero within 70 days. Natural infection from a prior case gave a bigger and more durable viral-load reduction than any number of vaccine doses [16]. A household study from Rio de Janeiro found that hybrid immunity (infection plus vaccination) cut transmission risk during Omicron by about half (OR 0.49), but vaccine-only immunity was not separately reported for that outcome [11]. A Chinese contact-tracing study of the inactivated vaccine BBIBP-CorV against BA.5 found that a booster reduced transmission by about 29% compared to two doses, with the effect waning after 90 days [1]. That is a modest, temporary reduction, not a block.

None of these studies compared vaccinated people to a true unvaccinated group for the transmission endpoint. The Chinese study compared booster recipients to two-dose recipients [1]. The Israeli study compared vaccinated to unvaccinated for viral load, not for actual secondary infections [16]. The Rio study compared hybrid immunity to no immunity [11]. The most informative design — randomize households, vaccinate half, follow everyone and test everyone regularly to see who infects whom — was never run. The evidence we have is observational, uses surrogate endpoints (viral load instead of actual transmission events), and comes from periods when the vaccinated and unvaccinated behaved differently (masking, testing, socializing). That makes the numbers unreliable for real-world decisions.

My call: COVID-19 vaccines produce a modest, short-lived reduction in viral load that likely lowers transmission risk for a few weeks after a dose, especially against earlier variants. Against Omicron, the effect is small and fades fast. The evidence does not support the claim that vaccination prevents transmission in any durable or reliable way. Confidence: moderate that the effect exists briefly, low that it matters for a person deciding whether they can safely be around others.