Austin Meyer, MD, PhD, MS, MPH, MS, FACP, FAAP

Baylor Scott & White Health

Baylor College of Medicine

Using surveillance and forecasting to optimize flu vaccine timing

UT Austin, Internal Medicine Grand Rounds

Disclosure

The University of Texas at Austin, Dell Medical School, Continuing Medical Education
The following speakers and planners do not have relevant financial relationship(s) with ineligible companies to disclose:
The planners of this educational activity have no relevant financial relationship(s) with ineligible companies to disclose.
The speaker for this educational activity has no relevant financial relationship(s) with ineligible companies to disclose.
The CME Advisory Committee, reviewers of this educational activity, have no relevant financial relationship(s) with ineligible companies to disclose.
The following speakers and planners have relevant financial relationship(s) with ineligible companies to disclose:
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Three questions for today

1What does flu surveillance data look like?
2How do we forecast the flu and how far ahead is it useful?
3When should patients get their flu vaccine?

The goal of respiratory virus surveillance is to use epidemic information to improve health

Simulated national influenza season
0%2%4%6%JunJulAugSepOctNovDecJanFebMarAprMayILI visitsSimulated onsetthreshold 2.5%
Jun 1, 20261.2% ILI
Aug 3, 20261.0% ILI
Sep 1, 20261.2% ILI
Nov 16, 20261.8% ILI
Nov 19, 20261.8% ILI
Nov 29, 20262.5% ILI
Dec 3, 20262.9% ILI
Dec 10, 20263.5% ILI
Dec 21, 20264.7% ILI
Jan 20, 20277.0% ILI
Jan 27, 20276.6% ILI
Feb 19, 20274.5% ILI
Mar 1, 20273.4% ILI
May 31, 20271.1% ILI
Below onset
Epidemic underway
Declining after peak
What surveillance shows Summer baseline The curve is quiet; preparation for the coming season starts before the epidemic signal appears.
Vaccine decision · Aug 3 Vaccine distribution begins Begin distributing vaccine to administration sites about four months before season onset.
Vaccine decision · Sep 1 Vaccination campaign starts Start the vaccination campaign before incidence begins rising to capture as many people as possible at outpatient points of care.
Vaccine decision · Nov 16 Target maximum uptake Aim to reach maximum vaccine uptake before the surveillance onset threshold is crossed.
Clinical operation · Nov 19 Stage antivirals at clinics Confirm antiviral supply and distribute stock to outpatient and acute-care sites.
Clinical operation · Nov 29 Adjust influenza pre-test probability As community influenza activity rises, update the pre-test probability used to interpret influenza testing results and guide empiric treatment decisions.
Surveillance milestone · Dec 3 Onset threshold crossed The national curve exceeds the surveillance onset threshold.
Public health guidance · Dec 10 Intensify prevention guidance Reinforce hand hygiene, staying home when ill, and masks or distance in crowded indoor spaces.
Clinical operation · Dec 21 Shift clinical resources Redistribute staffing, diagnostics, pharmaceuticals, and equipment toward regions experiencing the greatest burden.
Public health guidance · Jan 20 Peak: sustain interventions Communicate that activity is near its peak, and ask the public and health systems to sustain effective interventions.
Vaccine decision · Jan 27 Preliminary VE available Early vaccine-effectiveness estimates become available after most vaccination-timing decisions are already complete.
Vaccine decision · Feb 19 Select next season’s strains Select the Northern Hemisphere vaccine strains for the next influenza season.
Vaccine decision · Mar 1 Vaccine manufacturing begins Manufacturing for the next Northern Hemisphere season begins after U.S. strain selection in late February or early March.
Vaccine
Clinical
Public health
Surveillance

Decisions depend on the state of the epidemic, but what do we actually observe?

US influenza surveillance combines signals from different parts of the healthcare system

Surveillance along the patient journey
Patient journey
Specimen / laboratory signal
A person with influenza symptoms sneezing into a tissue
Symptomatic
illness
Outpatient
clinic
An outpatient clinic
ILINet
Virologic
testing
A clinical virology laboratory
ICLS
NREVSS
Emergency
department
A hospital emergency department entrance
NSSP
Hospital
admission
An inpatient hospital
FluSurv-NET
NHSN
Mortality
A respectful grave marker with flowers
NCHS
Pediatric deaths
Complementary signals
Outpatient and Emergency Care
Inpatient surveillance
Virologic surveillance
Mortality surveillance
Different systems answer different questions Illness, hospital care, laboratory testing, and deaths
Outpatient and Emergency Care ILINet · NSSP
Inpatient surveillance FluSurv-NET · NHSN
Virologic surveillance ICLS · NREVSS
Mortality surveillance NCHS · Pediatric deaths
Reports Outpatient and emergency visits, hospital admissions, laboratory results, and deaths.
Reports ILINet tracks visits for flu-like symptoms. NSSP tracks emergency visits with a flu diagnosis.
Reports FluSurv-NET tracks hospitalization rates and patient characteristics in selected communities. NHSN tracks admissions, occupied beds, and hospital capacity nationwide.
Reports ICLS focuses on influenza; NREVSS also tracks other respiratory and enteric viruses. Clinical and public-health labs report through these networks.
Reports NCHS tracks death-certificate diagnoses. Pediatric surveillance records individual confirmed flu deaths in children.
How to read it Use the signals together to follow flu activity and its impact on care. Each system measures a different event, so compare trends within each system.
How to read it Use them to follow demand for outpatient and emergency care. ILINet counts flu-like symptoms from any cause; NSSP counts visits diagnosed as flu.
How to read it Use FluSurv-NET rates to see who is hospitalized and NHSN counts to gauge pressure on hospitals. Both count patients admitted with a positive flu test.
How to read it Use clinical-lab positivity to follow flu activity among tested patients, and public-health results to see which flu viruses are circulating.
How to read it Use them to follow flu's fatal impact and who is affected. Deaths follow illness, and each week's figures update as reports arrive.
CDC surveillance methods ↗
Outpatient and Emergency Care · Outpatient clinic
ILINet U.S. Outpatient Influenza-like Illness Surveillance Network
Reports The weekly percentage of outpatient visits meeting the influenza-like illness definition, reported by age group.
How to read it Use it to follow when flu-like illness rises, peaks, and falls in outpatient clinics. It counts visits for fever with cough or sore throat, whatever the cause.
CDC surveillance methods ↗
Virologic surveillance · Influenza-focused network
ICLS U.S. Influenza Collaborating Laboratories System
Reports Weekly influenza tests and positive results. Public-health laboratories also report virus types, subtypes, lineages, and patient ages.
How to read it Use clinical-lab positivity to follow flu activity, and public-health results to see which flu viruses are circulating. Public-health labs identify each virus's type and subtype or lineage.
CDC surveillance methods ↗
Virologic surveillance · Multiple-virus network
NREVSS National Respiratory and Enteric Virus Surveillance System
Reports Weekly test totals and positive results for influenza and other respiratory and enteric viruses, from hospital, commercial, and public-health laboratories.
How to read it Use clinical-lab positivity to follow when and where flu is circulating. Positivity is the share of tested specimens that are positive for flu.
CDC surveillance methods ↗
Outpatient and Emergency Care · Emergency department
NSSP National Syndromic Surveillance Program
Reports The percentage of emergency-department visits with an influenza discharge diagnosis, overall and by age group.
How to read it Use it to follow flu trends in emergency departments, nationally and by state. A visit counts when its discharge diagnosis is influenza (ICD-10 J09–J11).
CDC surveillance methods ↗
Inpatient surveillance · Hospital admission
FluSurv-NET Influenza Hospitalization Surveillance Network
Reports Population-based weekly rates and detailed characteristics of laboratory-confirmed influenza hospitalizations in selected catchment areas.
How to read it Use it to compare hospitalization rates across ages and seasons and see who is most affected. Rates come from residents of more than 90 counties in 14 states.
CDC surveillance methods ↗
Inpatient surveillance · Hospital admission
NHSN National Healthcare Safety Network Hospital Respiratory Data Module
Reports Weekly facility-level counts of new laboratory-confirmed influenza admissions, current hospitalizations, ICU occupancy, and bed capacity.
How to read it Use it to track flu admissions and pressure on hospital and ICU beds, nationally and by state. Acute-care and critical-access hospitals are required to report weekly.
CDC surveillance methods ↗
Mortality surveillance · Death certificates
NCHS National Center for Health Statistics Mortality Surveillance
Reports Weekly death-certificate measures for deaths coded with influenza, pneumonia, and/or COVID-19.
How to read it Use flu-coded deaths to follow flu's fatal impact over the season. Each week's figures update as death certificates arrive.
CDC surveillance methods ↗
Mortality surveillance · Notifiable death
Pediatric deaths Influenza-Associated Pediatric Mortality Surveillance System
Reports Individual laboratory-confirmed influenza-associated deaths among people younger than 18 years, with demographic and clinical details.
How to read it Use it to track flu deaths in children, with each child's age, vaccination status, and underlying conditions. Season totals update as reports arrive.
CDC surveillance methods ↗

Each system covers different cohorts and has different biases

Poll question #1

https://www.meyerlab.io/ut-grand-rounds-visuals/poll-01.html?mode=admin

Poll question #1

ILINet has measured outpatient influenza-like illness nationwide since 1997/98

Outpatient clinics · ILINet
A monochrome illustration of an outpatient clinic
ILINet
U.S. Outpatient Influenza-like Illness Surveillance Network
Measures
% of outpatient visits for influenza-like illness: fever with cough or sore throat
Coverage
≈4,000 providers in all 50 states
CDC surveillance methods ↗
2024/25 · United States
1997/98–2025/26 · United States
2024/25 · 50 states + DC
0 22.34.544.59 6.16.813.58.1918 SepOctNovDecJan FebMarAprMay % outpatient visits for ILI United States 2024/25 Texas

NSSP grew from a bioterrorism early warning system into national emergency department surveillance

Emergency departments · NSSP
A monochrome illustration of an emergency department
NSSP
National Syndromic Surveillance Program
Measures
% of emergency department visits with a discharge diagnosis of influenza
Coverage
≈85% of U.S. emergency departments
CDC surveillance methods ↗
2024/25 · United States
2022/23–2025/26 · United States
2024/25 · 50 states + DC
0 2.22.33.84.44.57.5 6.66.811.38.8915 SepOctNovDecJan FebMarAprMay % ED visits with influenza diagnosis United States 2024/25 Texas

Flu and RSV joined COVID-19 in NHSN's required weekly count of lab-confirmed hospital admissions

Hospital admissions · NHSN
A monochrome illustration of an inpatient hospital
NHSN
National Healthcare Safety Network
Measures
New admissions with lab‑confirmed influenza per 100,000 people
Coverage
All acute‑care and critical‑access hospitals
CDC surveillance methods ↗
2024/25 · United States
2020/21–2025/26 · United States
2024/25 · 50 states + DC
0 4.510.3920.513.530.8 1841SepOctNov DecJanFebMarApr Mayadmissions per 100,000 United States 2024/25 Texas

FluSurv-NET has turned hospitalizations into population-based rates since 2003

Hospitalization rates · FluSurv-NET
A monochrome illustration of a hospital participating in a surveillance catchment
FluSurv-NET
Influenza Hospitalization Surveillance Network
Measures
Lab‑confirmed influenza hospitalizations per 100,000 residents
Coverage
More than 90 counties in 14 states
CDC surveillance methods ↗
2024/25 · all sites combined
2009/10–2025/26 · all sites combined
2024/25 · 9 states
0 3.867.51211.318 1524SepOctNov DecJanFebMar AprMayhospitalizations per 100,000 FluSurv-NET 2024/25 California

Clinical laboratories report how often tested specimens are positive for flu

Clinical laboratories · ICLS and NREVSS
A monochrome illustration of a clinical virology laboratory
ICLS · NREVSS
Clinical laboratories reporting through either network
Measures
% of tested specimens positive for influenza
Coverage
≈300 clinical labs in all 50 states
CDC surveillance methods ↗
2024/25 · United States
2016/17–2025/26 · United States
2024/25 · 43 states
0 8.512172425.536 3448SepOctNov DecJanFebMarApr May% clinical specimens positive United States 2024/25 Texas

When does flu season peak?

In each surveillance system,

the "typical" flu season looks different

In each surveillance system,

the "typical" flu season looks different

In each surveillance system,

the "typical" flu season looks different

In each surveillance system,

the "typical" flu season looks different

In each surveillance system,

the "typical" flu season looks different

How do we forecast the flu and

how far ahead is it useful?

Poll question #2

https://www.meyerlab.io/ut-grand-rounds-visuals/poll-02.html?mode=admin

Poll question #2

The CDC FluSight project

Many teams contribute to CDC's FluSight project

Models contributing regularly
Submitted forecasts in at least half of scheduled weeks · 2026/27 through Oct 24
23
21
34
43
48
59
2021/22
2022/23
2023/24
2024/25
2025/26
2026/27
Teams submit weekly forecasts
CDC produces a shared ensemble

Individual model forecasts vary widely

Texas · weekly influenza hospital admissions · December 11, 2024
ObservedIndividual modelsEnsemble median95% range01,0002,0003,0004,000Admissions per weekOct 5Oct 19Nov 2Nov 16Nov 30Dec 14Dec 28Jan 4Forecasts due
How many admissions should hospitals expect?
Forecasting teams
2024/25: 33 teams, 46 models
FluSight hub
Ensemble forecast
Hospital
Staffing · Beds · Supplies

The forecasting targets have changed over time

2013/14

Outpatient illness

2021/22

Hospitalizations

The forecasting targets have changed over time

2013/14

Outpatient illness

2021/22

Hospitalizations

2026/27 · Primary target

Weekly hospital admissions

Current week + 3 weeks ahead

The forecasting targets have changed over time

2013/14

Outpatient illness

2021/22

Hospitalizations

2026/27 · Primary target

Weekly hospital admissions

Current week + 3 weeks ahead

Flu ED visits

% of all ED visits

Peak timing

week of the year

Peak height

admission count

Hospitalization trend

5 categories

FluSight forecasts give a median and a range of likely admissions

Weekly laboratory-confirmed influenza hospital admissions
23 percentiles per location and target week02505007501,000Hospital admissionsObserved95% range50% rangeMedian5 weeks ago3 weeks agoLast weekCurrent week+1 week+2 weeks+3 weeksWISmeasures forecast error, penalizing wide ranges and misses.Lower is better.Relative WIS(rWIS) compares WIS with a forecast that repeats the latest count.Below 1 is better.

How our team forecasts flu hospitalizations

A new target, too little history

COVID-19 pandemic
Historical ILINet dataset
Sep 2009 – Sep 2020
Nationwide influenza
hospital admissions
Oct 2020 – Oct 2022
2009
2012
2015
2018
2020
2022
Forecasting begins
2022/23

Incomplete, optional flu reporting from 2020; mandatory from February 2022.

The history was short, and early reporting undercounted admissions

Texas · weekly influenza hospital admissions
05001,0001,500 2,0002,500Hospital admissions2009 2012201520182020 Oct 202201,0002,0003,000 4,0005,000Hospital admissions200920122015 201820212024Jul 2026 Reconstructed from ILINetObservedObserved, leveledA longer training history provides more context for the model to forecast.After quantile leveling: the training history for the 2026/27 season.Before leveling Meyer et al. · Epidemics · 2025

Our models moved from statistical to machine learning

Influenza hospitalization forecasting
StatisticalTime seriesState-specificLightGBMSeparate fitsEnsembling throughoutTexasCaliforniaNew YorkOther locationsPooledLightGBMShared trainingacross locationsFit the centerBest single estimateFit the spreadUncertainty around itTimeCenterSpreadDerive 23forecast percentiles

Forecasts are good at short horizon away from regime change

02,0004,0006,0008,00010,00012,00014,000Weekly influenza admissionsOct 52024Jan2025Apr2025Jul2025Oct2025Jan2026Apr 252026rWIS: 0.64rWIS: 0.60rWIS: 0.59rWIS: 0.60ObservedForecast median50% range90% range
Texas · MIGHTE-Base without additional covariatesForecast forThis weekForecast for1 week aheadForecast for2 weeks aheadForecast for3 weeks ahead

We are testing other surveillance signals as forecast inputs

Adding surveillance signals
HospitalizationsTexas · weekly admissions02,500HospitalizationsTexas · weekly admissions02,500HospitalizationsTexas · weekly admissions02,5005,000Influenza ED visitsU.S. · % of ED visits0510Influenza AwastewaterU.S. · log₁₀ normalized RNAInfluenza ED visitsU.S. · % of ED visits0510Influenza AwastewaterU.S. · log₁₀ normalized RNA-5-4-3RSV ED visitsU.S. · % of ED visits-5-4-300.61.2Oct 52024Jan2025Apr2025Jul2025Oct2025Jan2026Apr 252026

Adding wastewater lags does improve accuracy slightly

02,0004,0006,0008,00010,00012,00014,000Weekly influenza admissionsOct 52024Jan2025Apr2025Jul2025Oct2025Jan2026Apr 252026rWIS: 0.62rWIS: 0.58rWIS: 0.58rWIS: 0.59ObservedForecast median50% range90% range
Texas · MIGHTE-Base + wastewater lagsForecast forThis weekForecast for1 week aheadForecast for2 weeks aheadForecast for3 weeks ahead

We will post this season’s forecasts and rankings weekly

https://www.meyerlab.io/flusight_2026                           https://www.respilens.com

A forecast helps when it is tied to a planned action, such as staffing or resource allocation.
Forecasts are only good at short lead times; unreliable turning points.
Uneven partnerships with public health.
Limited connection and integration into healthcare operations.

Forecasting skill and operational usefulness are different goals

When should patients get their flu vaccine?

Poll question #3

https://www.meyerlab.io/ut-grand-rounds-visuals/poll-03.html?mode=admin

Poll question #3

The answer depends strongly on how protection wanes

Protection over timeMean modeled protectionRay et al. · 2019Adjusted odds of influenza+16% every 28 daysOR 1.16 (95% CI 1.13–1.20)1.0×1.5×2.0×2.5×14–4142–6970–9798–125126–153154+ReferenceDays after vaccination0%20%40%60%0481216202428Weeks after vaccinationLonger-lasting protection reduces the benefit of waiting.
1 · Start2 · Primary↺

The best week minimizes the burden left unprotected

Without vaccination, the full seasonal burden remainsAn early dose can wane before the winter peakA late dose leaves earlier illness unprotectedThe smallest shaded area identifies the best week2017/18 U.S. season · one illustrative protection curve
Vaccine protection0%30%60%2-week response lagWeekly share of seasonal outpatient burden0%2%4%6%2736451101926Before protectionBurden remainingMMWR week of yearVACCINATION WEEKNo vaccination100.0%Week 272017/18 season93.1%Week 282017/18 season92.6%Week 292017/18 season91.9%Week 302017/18 season91.1%Week 312017/18 season90.1%Week 322017/18 season89.1%Week 332017/18 season87.9%Week 342017/18 season86.6%Week 352017/18 season85.3%Week 362017/18 season83.9%Week 372017/18 season82.4%Week 382017/18 season81.0%Week 392017/18 season79.6%Week 402017/18 season78.3%Week 412017/18 season77.0%Week 422017/18 season75.8%Week 432017/18 season74.7%Week 442017/18 season73.7%Week 452017/18 season72.8%Week 462017/18 season72.1%Week 472017/18 season71.5%Week 482017/18 season71.1%Week 492017/18 season71.0%Week 502017/18 season71.6%Week 512017/18 season72.9%Week 522017/18 season74.4%Week 12017/18 season75.8%Week 22017/18 season77.6%Week 32017/18 season79.8%Week 42017/18 season82.4%Week 52017/18 season85.0%Week 62017/18 season87.2%Week 72017/18 season89.0%Week 82017/18 season90.3%Week 92017/18 season91.3%Week 102017/18 season92.2%Week 112017/18 season93.0%Week 122017/18 season93.7%Week 132017/18 season94.4%Week 142017/18 season95.0%Week 152017/18 season95.6%Week 162017/18 season96.1%Week 172017/18 season96.6%Week 182017/18 season97.0%Week 192017/18 season97.5%Week 202017/18 season98.0%Week 212017/18 season98.4%Week 222017/18 season98.8%Week 232017/18 season99.2%Week 242017/18 season99.6%Week 252017/18 season100.0%Week 262017/18 season100.0%of seasonal burden remainsShaded area · before protection = 100%Total burden remaining60%80%100%274011426Vaccination weekSmallest area in this exampleThe same epidemic.A different vaccination week.Compare the shaded areas.
Vaccination week1 · Start2 · Week 373 · Week 64 · Find minimum↺

We compare vaccination weeks across
many plausible seasons and waning scenarios

Start with one epidemic and protection curveOne possible epidemic and protection curveThe same comparison across sampled scenariosChoose the week with the lowest average burden1 of 5,000 scenarios shown2 of 5,000 scenarios shown3 of 5,000 scenarios shown4 of 5,000 scenarios shown5 of 5,000 scenarios shown6 of 5,000 scenarios shown7 of 5,000 scenarios shown8 of 5,000 scenarios shown9 of 5,000 scenarios shown10 of 5,000 scenarios shown
Possible influenza seasonsWeekly ILI · MMWR week2024/252023/242018/192017/182017/182011/122024/252012/132025/262011/120%4%8%274011426One dose · MMWR week 27One dose · MMWR week 28One dose · MMWR week 29One dose · MMWR week 30One dose · MMWR week 31One dose · MMWR week 32One dose · MMWR week 33One dose · MMWR week 34One dose · MMWR week 35One dose · MMWR week 36One dose · MMWR week 37One dose · MMWR week 38One dose · MMWR week 39One dose · MMWR week 40One dose · MMWR week 41One dose · MMWR week 42One dose · MMWR week 43One dose · MMWR week 44One dose · MMWR week 45One dose · MMWR week 46One dose · MMWR week 47One dose · MMWR week 48One dose · MMWR week 49One dose · MMWR week 50One dose · MMWR week 51One dose · MMWR week 52One dose · MMWR week 1One dose · MMWR week 2One dose · MMWR week 3One dose · MMWR week 4One dose · MMWR week 5One dose · MMWR week 6One dose · MMWR week 7One dose · MMWR week 8One dose · MMWR week 9One dose · MMWR week 10One dose · MMWR week 11One dose · MMWR week 12One dose · MMWR week 13One dose · MMWR week 14One dose · MMWR week 15One dose · MMWR week 16One dose · MMWR week 17One dose · MMWR week 18One dose · MMWR week 19One dose · MMWR week 20One dose · MMWR week 21One dose · MMWR week 22One dose · MMWR week 23One dose · MMWR week 24One dose · MMWR week 25One dose · MMWR week 26Possible vaccine protection0%40%80%0122436Weeks since vaccinationCompare everyvaccination weekTiming regretOne scenarioTen example scenariosAverage of all 5,000 scenariosMore0Lowest average burden274011426Vaccination week (MMWR)Each week gives one pointEach pairing gives a different curveOne week chosen before the season
1 · Start2 · One scenario3 · Sample scenarios4 · Average outcomes↺

After 5000 scenario draws we can predict the optimal week

Burden left avoidable because of timingSeasonal ILI visits per 10,000 outpatient encounters each weekGuidance · weeks 36–4405001,0001,5002,0002,5003,0002736451101926Week 40512Week 44192Near-optimal windowWeeks 47–48≈53Week 48 minimumVaccination week (MMWR)Lower is betterMean of 5,000 scenariosNational · All ages
1 · Start2 · Show the result3 · Highlight the window↺

Timing changes how much protection we lose

Protection lost compared with the best weekNational · All ages0%5%10%15%20%25%Earlier vaccinationWeek 40End of guidanceWeek 44Selected fixed weekWeek 4821.9%9.2%2.3%Relative to the best vaccination week in each scenario
1 · Start2 · Week 403 · Week 444 · Week 48↺

A later date only helps if vaccination still happens

At today's visitOne patient Vaccinate todaySchedule a later dose Dose receivedAssumed in the studyVaccination missed Dose receivedIf a later visit is uncertain, vaccinate today.CDC: September–October for most one-dose recipients; avoid missed opportunities.
1 · Start2 · Study assumption3 · A missed dose4 · Clinical decision↺

Rather than a best fixed week, is there a tool that is flexible to seasonal variation?

A threshold might be useful to identify the best week

NEXT STUDY1 of 5,000 pre-pandemic pairs2 of 5,000 pre-pandemic pairs3 of 5,000 pre-pandemic pairs4 of 5,000 pre-pandemic pairs5 of 5,000 pre-pandemic pairs6 of 5,000 pre-pandemic pairs7 of 5,000 pre-pandemic pairs8 of 5,000 pre-pandemic pairs9 of 5,000 pre-pandemic pairs10 of 5,000 pre-pandemic pairs11 of 5,000 pre-pandemic pairs12 of 5,000 pre-pandemic pairs13 of 5,000 pre-pandemic pairs14 of 5,000 pre-pandemic pairs15 of 5,000 pre-pandemic pairs16 of 5,000 pre-pandemic pairs17 of 5,000 pre-pandemic pairs18 of 5,000 pre-pandemic pairs19 of 5,000 pre-pandemic pairs20 of 5,000 pre-pandemic pairs21 of 5,000 pre-pandemic pairs22 of 5,000 pre-pandemic pairs23 of 5,000 pre-pandemic pairs24 of 5,000 pre-pandemic pairs25 of 5,000 pre-pandemic pairs26 of 5,000 pre-pandemic pairs27 of 5,000 pre-pandemic pairs28 of 5,000 pre-pandemic pairs29 of 5,000 pre-pandemic pairs30 of 5,000 pre-pandemic pairs31 of 5,000 pre-pandemic pairs32 of 5,000 pre-pandemic pairs33 of 5,000 pre-pandemic pairs34 of 5,000 pre-pandemic pairs35 of 5,000 pre-pandemic pairs36 of 5,000 pre-pandemic pairs37 of 5,000 pre-pandemic pairs38 of 5,000 pre-pandemic pairs39 of 5,000 pre-pandemic pairs40 of 5,000 pre-pandemic pairs41 of 5,000 pre-pandemic pairs42 of 5,000 pre-pandemic pairs43 of 5,000 pre-pandemic pairs44 of 5,000 pre-pandemic pairs45 of 5,000 pre-pandemic pairs46 of 5,000 pre-pandemic pairs47 of 5,000 pre-pandemic pairs48 of 5,000 pre-pandemic pairs49 of 5,000 pre-pandemic pairs50 of 5,000 pre-pandemic pairsPre-pandemic seasons · 2011–1950 of 5,000 later-season pairs · 2023–26Possible influenza seasonsWeekly ILI · MMWR week2018/192016/172015/162012/132016/172016/172011/122016/172018/192014/152013/142012/132014/152017/182016/172011/122014/152018/192018/192016/172013/142011/122013/142012/132017/182011/122013/142011/122011/122015/162011/122014/152017/182014/152015/162011/122018/192011/122014/152011/122011/122016/172018/192016/172014/152015/162018/192014/152018/192013/140%4%8%274011426Vaccine protectionAligned to each optimal vaccination week0%40%80%274011426Compare everyvaccination weekTiming regretBurden avoidable by changing the dose dateMore0274011426SeasonsAligned meanOptimal weeks01.252.5ILI above summer baseline / historical seasonal rise0.38-32-24-16-8Peak+8+16+24Weeks from the aligned mean peakVaccinate above summer baseline by0.38of the historical seasonal riseFrom 5,000 paired drawsLater seasonsOptimal weeksNational trigger weeks01.252.5ILI above summer baseline / historical seasonal rise274011426US: W43 / W47Fixed Week 48MMWR week of yearMedian gap to optimumWeek 48National1 wk1 wkBenefit vs. optimum97.7%98.3%Across all 5,000 pairsNEXT STUDY2023–26 · 50 of 5,000 draws per stateStandardized ILIOptimal weeksNational trigger weeks2023/24: W43 · 2024/25–2025/26: W47Texas012274011426US: W43 / W47Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 443 wk96.6%FixedWeek 482 wk96.1%NationalW43 / W471 wk98.8%California01.53274011426US: W43 / W47Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 443 wk96.9%FixedWeek 482 wk97.2%NationalW43 / W471 wk99.0%Minnesota01.753.5274011426US: W43 / W47Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 445 wk90.5%FixedWeek 481 wk98.4%NationalW43 / W472 wk95.7%Massachusetts04.258.5274011426US: W43 / W47Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 444 wk89.7%FixedWeek 481 wk97.9%NationalW43 / W473 wk95.0%
1 · Start2 · One scenario3 · Sample scenarios4 · Before alignment5 · Align seasons6 · Test a threshold7 · States: W448 · + W489 · + threshold↺

Can the same rule adapt to an early season?

NEXT STUDYUS · 2022–23 · 50 of 5,000 paired draws2022–23 · 50 of 5,000 draws per stateStandardized ILIOptimal weeksNational trigger weeks012274011426US: Week 41Week 44Week 48Median gap to optimumBenefit vs. optimumAcross all 5,000 pairsGuidanceWeek 443 wk91.7%FixedWeek 487 wk68.6%NationalWeek 411 wk99.1%MMWR week of yearTexas012274011426US: Week 41Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 445 wk83.9%FixedWeek 489 wk61.9%NationalWeek 412 wk96.8%California01.53274011426US: Week 41Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 442 wk97.0%FixedWeek 486 wk78.1%NationalWeek 411 wk98.5%Minnesota01.252.5274011426US: Week 41Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 443 wk93.4%FixedWeek 487 wk62.0%NationalWeek 411 wk99.2%Massachusetts03.256.5274011426US: Week 41Week 44Week 48Median gap to optimumBenefit vs. optimumGuidanceWeek 441 wk99.0%FixedWeek 484 wk79.4%NationalWeek 413 wk96.2%
1 · Start2 · US: Week 443 · + Week 484 · + threshold5 · States: Week 446 · + Week 487 · + threshold↺

How can forecasts improve our vaccine decisions?

This week's forecast is set to cross the vaccination threshold

CURRENT ILINETVisits for ILI (%)Forecast median · 80% rangeVaccination ThresholdNational trigger weekUnited States04.759.5W27 2025W01 2026W27 2026W45 20262.22%Projected national trigger: W42 · Oct 24Texas04.759.5W27 2025W01 2026W27 2026W45 2026US week 42California03.57W27 2025W01 2026W27 2026W45 2026US week 42Minnesota05.2510.5W27 2025W01 2026W27 2026W45 2026US week 42Massachusetts0714W27 2025W01 2026W27 2026W45 2026US week 42
1 · US2 · Forecast3 · Threshold4 · States5 · Forecasts6 · National week↺

What to take back to clinical work

1Each surveillance system measures a different step of illness; together they show when flu rises, peaks, and falls.
2Forecasts are most accurate in the near term and away from turning points.
3Our national model favors MMWR week 48 for a single dose; it is likely that a data driven approach is better; when a later visit is uncertain, vaccinate today.

Thanks for your attention

Mauricio Santillana
Mauricio Santillana
Northeastern University
Research mentor
Shihao Yang
Shihao Yang
Georgia Tech
Leo Clemente
Leo Clemente
Santillana Lab
Fred Lu
Fred Lu
Santillana Lab
Candice Djorno
Candice Djorno
Yang Lab
 
What questions do you have?
Clay community overview of U.S. influenza surveillance