One injection, not five, and not only for premature babies

The drug that came before nirsevimab, palivizumab, worked. It was also
**recommended only for children with certain underlying medical conditions —
about 5% of all infants** — and **use was further limited by high cost and the
requirement for monthly injections** through the season.

That left out the group where most of the hospitalisations actually happen.

Why the narrow group was the wrong group

RSV infection is the leading cause of hospitalisation among US infants.
**Most children are infected during the first year of life, and nearly all
have been infected by age 2.** Each year brings roughly **50,000–80,000
RSV-associated hospitalisations** and 100–300 RSV-associated deaths among
US infants and young children.

Prematurity is a real risk factor: **the rate of RSV-associated hospitalisation
among infants born at ≤30 weeks' gestation is three times that of term
infants**, with higher rates of intensive care admission too.

But:

**RSV is also the leading cause of hospitalisation among healthy term
infants.**

A product reserved for the highest-risk 5% cannot change a number driven by
the other 95%.

What ACIP recommended, on 3 August 2023

One intramuscular injection, shortly before or during the RSV season —
typically autumn through spring — in place of monthly ones.

The evidence

Among infants aged <8 months in their first season, pooled results from a
phase 2b trial of 1,453 preterm infants (29–34 weeks) and a **phase 3
trial of 3,012 late preterm and term infants (≥35 weeks)**, followed for 150
days:

No RSV deaths in either trial, and **serious adverse events were not
increased** against placebo. GRADE certainty: moderate.

That ICU confidence interval — 16.4% to 98.8% — is worth reading rather
than skipping. One child in the nirsevimab arm was admitted to intensive care
against six on placebo. The effect is almost certainly real and the size of it
is almost entirely unknown, because the event is rare.

Where the cost argument turns

Fifteen times the cost for a fifteenth of the benefit, because **children
entering their second RSV season are at reduced risk** to begin with. For the
children who would have had palivizumab, though, nirsevimab is **expected to
be cost saving** — one injection instead of five.

Timing, and the places the calendar does not fit

Nirsevimab goes in shortly before the season starts — on pre-pandemic
patterns, October through the end of March across most of the continental
United States.

Two exceptions are written into the recommendation rather than left to be
discovered:

• Tropical climates — southern Florida, Guam, Hawaii, Puerto Rico, the
  US-affiliated Pacific Islands and the US Virgin Islands — where seasonality
  might differ or be unpredictable
• Alaska, where seasonality is **less predictable and the season is often
  longer than the national average**

Providers there should consult state, local or territorial guidance.

And one thing it is not for: **no evidence supports use against
hospital-acquired RSV infection, and it is not recommended for that.**

Source: Centers for Disease Control and Prevention, MMWR.