The 2019–20 influenza season **crossed the national baseline in the week
ending 9 November 2019** — **the earliest start since the 2009 A(H1N1)
pandemic** — and was still elevated in mid-February.
From 4,112 children and adults enrolled in the US Flu VE Network between
23 October 2019 and 25 January 2020:
| Against | Vaccine effectiveness | 95% CI |
|---|---|---|
| Any influenza virus | 45% | 36–53 |
| Influenza B/Victoria | 50% | 39–59 |
| Influenza A(H1N1)pdm09 | 37% | 19–52 |
| Children and adolescents aged 6 months–17 years | 55% | 42–65 |
How to read 45%
It is not a school mark. It means **45% fewer medical visits for
laboratory-confirmed influenza** among vaccinated people than among unvaccinated
ones — nearly half the doctor's visits that would have happened, prevented.
And it is normal:
**Interim VE estimates are consistent with those from previous seasons,
ranging from 40%–60% when influenza vaccines were antigenically matched to
circulating viruses.**
45% is a matched vaccine performing as a matched vaccine does. The flu vaccine
is not a poor version of the measles vaccine; it is an annual product against
a target that changes, and this is what success looks like.
What is measured, and what is not
**Medically attended acute respiratory illness with laboratory-confirmed
influenza** — someone ill enough to see a doctor, who was then tested. It does
not measure hospitalisations or deaths, which is the outcome most people care
about, and for which protection is generally higher than for illness overall.
The children's figure of 55% is the highest in the table, and it is the
group that spreads influenza most.
The recommendation that follows from the timing
Because the season started early, the natural question is whether it is too
late to vaccinate. The answer is no, for two distinct reasons:
**CDC recommends that health care providers continue to administer influenza
vaccine to persons aged ≥6 months because influenza activity is ongoing, and
the vaccine can still prevent illness, hospitalization, and death associated
with currently circulating influenza viruses as well as other influenza
viruses that might circulate later in the season.**
A season is not one virus. B/Victoria led this one; the A(H1N1) wave that may
follow is a second opportunity for a vaccine given in February to matter.
Source: Centers for Disease Control and Prevention, MMWR.
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