A safer-to-make polio shot matched the standard one in a trial of Nepali infants

A polio vaccine made from weakened Sabin virus strains protected infants about as well as the conventional vaccine made from wild-type Salk strains, according to a randomised trial published on 5 November in The Lancet Global Health [s1]. The result is narrow in scope but strategically important: it supports a shift in how the world manufactures one of its most consequential vaccines.

The distinction sounds technical, and it is, but it sits at the centre of the campaign to eradicate polio. Inactivated poliovirus vaccine (IPV) has traditionally been grown from Salk strains — fully virulent wild polioviruses killed before they go into the vial. Making it therefore requires handling large volumes of dangerous virus in high-containment facilities. Sabin strains are the attenuated, weakened viruses long used in the oral polio vaccine; growing IPV from them lowers the biosafety burden of production. As the world moves toward a polio-free future, the argument for not manufacturing vaccine from live wild virus becomes stronger.

What the trial did

The study was a single-centre, double-blind, randomised, controlled non-inferiority trial run at Siddhi Memorial Hospital in Bhaktapur, Nepal [s1]. Healthy infants aged 6 to 8 weeks, weighing at least 3.0 kg, were eligible.

A total of 460 infants were randomly assigned 1:1 to receive either Sabin IPV (made by LG Chem, South Korea) or Salk IPV (made by Bilthoven Biologicals, Netherlands), 230 in each group [s1]. Each 0.5 mL dose was given intramuscularly at 14 weeks and again at 9 months of age. All infants also received bivalent oral polio vaccine at 6, 10 and 14 weeks as part of Nepal's routine schedule — so the trial tested a real-world sequential schedule, not an idealised one [s1].

Parents, investigators and laboratory staff were masked to which IPV each infant received; only the vaccinators knew [s1]. The primary outcome was type 2 seroconversion — a rise in neutralising antibodies from below the protective threshold to at or above it — measured at 10 months, four weeks after the second IPV dose. The pre-specified non-inferiority margin was −10 percentage points.

What it found

Type 2 seroconversion occurred in 210 of 212 infants in the Sabin IPV group (99%, 95% CI 97 to 100) and 214 of 215 in the Salk IPV group (100%, 95% CI 97 to 100) [s1]. The unadjusted risk difference was −0.5 percentage points (95% CI −2.1 to 1.1) — comfortably inside the −10-point margin, meeting the trial's definition of non-inferiority [s1].

Safety looked similar between the two vaccines. The most common adverse event was fever after the 14-week visit, reported in 43% of Sabin IPV infants and 45% of Salk IPV infants [s1]. Across the trial there were 42 serious adverse events in 37 of 459 infants (8%), none of which investigators judged to be caused by either vaccine [s1].

How to read it

Non-inferiority is a specific and limited claim. The trial was not designed to show that Sabin IPV is better than Salk IPV — only that it is not meaningfully worse, within a margin agreed in advance. On that question the answer is clean: two doses of Sabin IPV, given alongside routine oral vaccine, produced type 2 protection on a par with the established product.

Type 2 is the strain the trial was powered around, and for good reason — it is the type behind most circulating vaccine-derived poliovirus outbreaks today, and the one the eradication programme watches most closely. The study was a single site in one city, so it speaks to immunogenicity and short-term safety rather than to field effectiveness across varied settings, and the trial was funded by the World Health Organization [s1][s2].

What to watch

The practical value here is supply. A vaccine that performs like the standard one but can be made without culturing live wild poliovirus is easier and safer to produce at scale — exactly what a programme trying to retire wild virus from the planet needs. The questions that remain are the ones a single immunogenicity trial cannot answer: performance in other populations, durability of protection, and how quickly manufacturers can bring Sabin-based IPV into routine use. For context, see our coverage of poliovirus eradication and of inactivated polio vaccine supply.

This article describes trial results. It is not medical advice, and vaccination decisions belong with a clinician and the relevant immunisation schedule.