A quarter-dose polio vaccine matched the full dose in Indian infants

Design

phase 2/3, double-blind, randomised controlled non-inferiority trial at 9 Indian tertiary hospitals, margin 10%

Population

643 healthy Indian infants aged 6-8 weeks who had received a birth dose of bivalent OPV

Primary outcome

type-specific seroconversion 28 days after the third dose, per-protocol

Effect

type 1 94.7% vs 92.8% (difference 1.9%, 95% CI −2.1 to 5.8); type 2 96.3% vs 97.9% (−1.6%, −4.7 to 1.5); type 3 97.3% vs 99.0% (−1.6%, −3.8 to 0.5)

A phase 2/3 double-blind trial at nine tertiary hospitals in India randomised 643 infants, all of whom had received a birth dose of bivalent oral polio vaccine, to an adjuvanted dose-sparing inactivated poliovirus vaccine with about a quarter of the antigen content, or to full-dose IPV, given intramuscularly at 6, 10 and 14 weeks alongside the routine schedule.

Seroconversion 28 days after the third dose was non-inferior for all three serotypes, against a 10% margin. Type 1: 94.7% against 92.8% (difference 1.9%, 95% CI −2.1 to 5.8). Type 2: 96.3% against 97.9% (−1.6%, −4.7 to 1.5). Type 3: 97.3% against 99.0% (−1.6%, −3.8 to 0.5). Local reactions and fever were common in both groups; no serious adverse events were attributed to either vaccine.

The significance here is supply, not immunology. IPV antigen is the constraint on polio campaigns in exactly the countries that need them, and fractional-dose intradermal schedules were adopted as a workaround with real delivery problems — intradermal injection needs trained staff and is harder to do at volume. An adjuvanted intramuscular quarter-dose sidesteps that. This is an immunogenicity endpoint, not disease prevention, which is the standard basis for polio vaccine licensure but worth naming.

• Seroconversion is the licensure endpoint for polio vaccines; no efficacy trial exists or is expected
• All infants here had a birth dose of bivalent OPV — the result belongs to that schedule
• Intramuscular delivery avoids the staff training problem that has dogged fractional intradermal IPV
• Expect this to matter for supply and campaign planning before it changes any individual prescription

Why it matters

Antigen supply, not efficacy, is what limits polio vaccination in high-burden countries, and this addresses supply without the intradermal delivery problem.

Don't overread it

Antibody titres are a surrogate — this trial did not and could not measure protection against paralytic disease.

The statistics, in plain English

Non-inferiority means the new vaccine is not worse by more than a prespecified margin — here 10 percentage points of seroconversion. For type 3 the difference was −1.6% with a lower bound of −3.8%, comfortably inside that margin, so the claim holds. It does not mean the vaccines are identical, and a margin of 10% is a policy judgement rather than a biological one.

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