Waiting for the data to arrive means finding out after it mattered

Surveillance data are often subject to delays, which is a problem for
anyone trying to decide today what is happening today.

A case reported this week was an infection a fortnight ago. The curve you can
see is always the curve as it was, and by the time it clearly turns, the turn
happened weeks back.

Nowcasting

Nowcasting estimates what the current numbers will look like once the
delayed reports arrive — correcting for reporting lag rather than waiting it
out.

Applied to Rt, the reproduction number, it produces **epidemic trend
categories** that say whether transmission is growing or declining now
rather than a fortnight ago.

What happened in summer 2024

**During summer 2024, trend categories that incorporated nowcasting provided
an early indication that COVID-19 community transmission was increasing
and later, provided confirmation that community transmission was
decreasing.**

Both halves matter and the second is the less obvious one. Telling a health
department that a wave has started is valuable; telling them with confidence
that it is over is what lets resources be released, and that decision is
usually made late and nervously.

Why decision-makers wanted this

**Epidemic trend categories that incorporate nowcasting can provide early
indication of changing trends in community transmission and can help public
health decision-makers quickly determine whether transmission of infection
is growing or declining at a given time and prepare for public health
action.**

The word doing the work is prepare. A signal that arrives after the peak
is a historical record; one that arrives during the climb is an operational
instruction.

*Source: Richard DM, Susswein Z, Connolly S, et al. Detection of Real-Time
Changes in Direction of COVID-19 Transmission Using National- and State-Level
Epidemic Trends Based on Rt Estimates — United States Overall and New Mexico,
April–October 2024. MMWR, Centers for Disease Control and Prevention.*