A quarter-dose polio shot made in India matches full IPV in an infant trial

The inactivated polio vaccine works, but it is expensive to make and hard to deliver in campaigns, and both problems come down to how much antigen each dose needs. A trial in India tested a vaccine built to need less of it — and found the smaller dose held its own.

The vaccine, an adjuvanted dose-sparing IPV, or ds-IPV, was developed in India with roughly a quarter of the antigen content of a full IPV dose [s1]. Its makers ran a phase 2/3, double-blind, randomised controlled trial at nine tertiary-care hospitals to see whether that reduced formulation could match standard IPV in infants [s1]. The result, published in The Lancet Infectious Diseases, was non-inferiority across all three poliovirus serotypes [s1].

The trial

Healthy infants aged six to eight weeks who had received a birth dose of bivalent oral polio vaccine were enrolled [s1]. Between 23 May 2022 and 13 April 2024, 658 were screened, 648 were eligible and randomly assigned one-to-one to ds-IPV or standard IPV, and 643 went on to receive vaccine — 323 the dose-sparing formulation and 320 the standard one, after five withdrew consent [s1]. Each infant got three intramuscular doses, at six, ten and 14 weeks, alongside the routine schedule of oral rotavirus vaccine and injectable DTwP-HB-Hib and pneumococcal conjugate vaccine given in the other thigh [s1]. Masking was tight enough that syringes were covered with an opaque peel before administration [s1].

Blood was drawn before the first dose and 28 days after the third, and neutralising antibodies against each serotype were measured by microneutralisation assay [s1]. The primary outcome was type-specific seroconversion, with a non-inferiority margin of 10 percentage points [s1].

The results

Seroconversion after three doses was high in both arms. For type 1, 94.7% of ds-IPV recipients seroconverted versus 92.8% for IPV, a difference of 1.9 points (95% CI −2.1 to 5.8) [s1]. For type 2, the figures were 96.3% versus 97.9%, a difference of −1.6 points (−4.7 to 1.5) [s1]. For type 3, 97.3% versus 99.0%, again a difference of −1.6 points (−3.8 to 0.5) [s1]. In every case the confidence interval stayed within the pre-specified margin, meeting the non-inferiority bar [s1].

Safety looked routine. Solicited reactions — tenderness, redness, swelling and fever — were very common, occurring in at least one in ten infants in both groups, and no serious adverse events were judged causally related to either vaccine [s1]. The authors conclude that ds-IPV was immunologically non-inferior to IPV with a similar safety profile, and could become an alternative option that helps keep IPV in steady supply for both poliovirus-naive and previously exposed populations [s1]. The trial was funded by the Serum Institute of India [s1].

Why a smaller dose matters

The appeal is not the immune response — full IPV already produces one — but the arithmetic behind it. Several countries already stretch limited IPV supply by giving fractional or reduced-dose regimens alongside oral vaccine, precisely because the full injectable dose is costly to procure and awkward to use in mass campaigns [s1]. A vaccine engineered to need about a quarter of the antigen, if it clears regulatory review, would let the same quantity of antigen protect more children, easing a supply constraint that has shaped polio strategy for years.

The trial also matters for who ran it and where. It was conducted entirely in India, in infants who had already received a birth dose of oral vaccine, and measured beyond seroconversion to the secondary outcomes of type-specific geometric mean titres and seroprotection, defined as a titre of eight or above [s1]. Immunogenicity was judged in the per-protocol population as the primary analysis, with the full analysis population as support [s1]. That design — a locally developed vaccine tested against the international standard in the exact population that would use it — is the kind of evidence procurement agencies need before a new product can enter routine schedules.

That constraint sits against an eradication effort that is nearly, but not quite, finished. WHO's fact sheet records that wild poliovirus cases have fallen by more than 99% since 1988, from an estimated 350,000 cases across more than 125 endemic countries to just two endemic countries [s2]. Polio mainly affects children under five, one in 200 infections leads to irreversible paralysis, and 5–10% of those paralysed die when their breathing muscles are immobilised [s2]. As long as a single child remains infected, WHO warns, children everywhere are at risk [s2].

The closing stretch of eradication depends on immunising the hardest-to-reach children reliably, which is where cost and logistics bite hardest. A dose-sparing vaccine does not change the immunology of polio protection. It changes how far a fixed supply of it can go — and in the final phase of an eradication programme, that is the variable under strain.

Sources

• Immunogenicity and safety of a dose-sparing inactivated poliovirus vaccine in infants in India — The Lancet Infectious Diseases, September 2026
• Poliomyelitis (fact sheet) — World Health Organization, 27 March 2019