One pneumococcal dose plus a booster still holds at five and a half years

Design

long-term follow-up of a cluster-randomised controlled trial, 24 communes randomised 1:1:1:1, plus three unvaccinated communes

Population

49,644 participants in Nha Trang, Viet Nam — 10,423 infants, 10,988 toddlers, 9,580 preschool children, 18,653 caregivers

Primary outcome

vaccine-type nasopharyngeal carriage prevalence at 5.5 years, non-inferiority margin 5 percentage points

Effect

infants 1p+1 vs 2p+1 difference −1.2 pp (95% CI −3.0 to 0.6); vs 3p+0 −0.2 pp (−1.7 to 1.4); vaccine-type carriage among carriers fell from 39.4% to 10.5% in adult caregivers

Reduced-dose pneumococcal conjugate schedules save money and delivery visits, and the question has always been whether the protection lasts and whether herd effects survive. This cluster-randomised trial in 24 communes of Nha Trang, Viet Nam, assigned infants to one of four PCV10 schedules — a single 12-month dose, one primary dose plus a 12-month booster, two primary doses plus a booster, or three primary doses with no booster — with three unvaccinated communes for comparison, and followed 49,644 participants to 5.5 years after introduction.

Vaccine-type carriage at 5.5 years was 0.7% in infants on the one-plus-one schedule, 1.9% on two-plus-one and 0.9% on three-plus-zero; in toddlers, 0.9%, 0.6% and 1.0%. One-plus-one met non-inferiority against both three-dose schedules in every infant and toddler comparison, against a 5 percentage-point margin. The single-dose 0p+1 schedule met non-inferiority in seven of eight intention-to-treat comparisons.

The indirect effect is the part that matters for programmes. Among pneumococcal carriers, the proportion carrying vaccine-type serotypes fell from 52.1% to 7.6% in infants, 50.0% to 4.0% in toddlers and 39.4% to 10.5% in adult caregivers — at similar rates across all schedule groups. A reduced schedule generated the same population-level protection as a full one.

• This applies to settings with an established programme after a catch-up campaign, not to introducing PCV into a naive population.
• The endpoint is nasopharyngeal carriage, not invasive pneumococcal disease — carriage is the driver of transmission and a validated proxy, but it is a proxy.
• PCV10 is the vaccine tested; serotype coverage differs for PCV13 and the higher-valent products now available, and carriage replacement patterns differ with it.
• India introduced PCV into the universal programme on a 2p+1 schedule; a move to 1p+1 would free a visit and a dose per child at national scale, which is the practical significance here.
• Fifty serious adverse events occurred within a month of vaccination across six years and none were attributed to the vaccine.

Why it matters

It turns a reduced pneumococcal schedule from a cost-saving proposal into a choice with five and a half years of randomised carriage data behind it.

Don't overread it

Carriage is a proxy for transmission — this trial did not measure invasive pneumococcal disease.

The statistics, in plain English

Non-inferiority was set at 5 percentage points, and the observed differences were all within about 1.2 points with intervals well inside the margin — so this is a comfortable pass rather than a marginal one. But note how low the event rates are: 2 carriers out of 286 infants. When prevalence falls that far, the trial is no longer able to detect small differences between schedules, and non-inferiority becomes easy to meet for a reason that is partly success and partly lost precision.

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