Neutralising titre explains part of protection from severe COVID-19, not all of it

Design

systematic review and meta-regression matching clinical outcome studies to neutralisation titre studies

Population

25 clinical studies of progression to severe COVID-19 after SARS-CoV-2 infection, matched to 301 post-vaccination neutralisation titre studies across variants

Primary outcome

vaccine protection against progression to hospital or intensive care admission, as a function of neutralising titre

Effect

relative risk 0.57 per ten-fold increase in geometric mean titre (95% CI 0.51 to 0.64), p<0.001

A meta-regression brought together 25 clinical studies of vaccine protection against progression from SARS-CoV-2 infection to hospital or intensive care admission, and matched them to 301 studies of post-vaccination neutralisation titres against various variants drawn from the Stanford coronavirus resistance database.

Neutralising antibody titre was associated with protection from progression: relative risk 0.57 per ten-fold rise in geometric mean titre (95% CI 0.51 to 0.64, p<0.001). But vaccination protected against progression better than therapeutic passive antibody given to people with confirmed infection - a gap the authors read as evidence that other immune responses, presumably cellular, carry part of the load.

For a clinician this is mechanism rather than management, and it should be read as such. Its practical edge is in what it warns against: using a neutralising titre as a bedside test of whether an individual patient is protected from severe disease. The correlation is at population level, across studies that measured titres and outcomes in different people, with the authors themselves naming unmeasured confounding as the main limitation. It also explains why protection against severe outcomes has held up better than protection against infection as titres wane against new variants.

• Do not order or interpret a neutralising antibody titre to decide whether an individual is protected - this is a population-level correlate.
• Expect protection against severe disease to outlast protection against infection, and say so when patients ask why they caught it anyway.
• Weigh passive antibody products on their own trial evidence, not on titre equivalence to vaccination.
• Note the design limit: titres and outcomes came from different studies, so this is an ecological correlation, not a within-person one.

The statistics, in plain English

A relative risk of 0.57 per ten-fold rise in titre describes a dose-response gradient, which is one of the stronger arguments for a causal contribution - but the titres and the clinical outcomes were measured in different studies and then matched, which is an ecological design. Correlations at study level can be much stronger than the same relationship within individuals. The confidence interval, 0.51 to 0.64, is tight, reflecting the number of studies pooled rather than the certainty of the underlying mechanism. The gap between vaccine-induced and passively administered antibody protection is the finding that stops titre being read as the whole explanation.

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