Flu positivity rose to 1.8% and measles reached 3,471: what to book this week
This week's flu report shows the season's first real uptick at 1.8% test positivity, measles reached 3,471 cases even as the weekly curve slowed, and last season's effectiveness figures show what this year's updated trivalent shot can realistically do.
The new flu season showed up in the numbers this week. Of 28,999 respiratory specimens tested by clinical laboratories nationwide during the week ending September 12, 514 came back positive for influenza — 1.8%, up from 1.3% the week before. 1
Confirmed measles cases reached 3,471 as of September 17, an increase of 177 in one week. 2
Those two facts set the household calendar for the next six weeks. Protection from a flu shot takes about two weeks to build, and CDC keeps the target for everyone at the end of October, so an appointment made this week lands comfortably inside the window. 3 Measles is the slower problem: the count keeps climbing, cases remain concentrated in a few states, and the record check families were asked to make in August is still the one that matters.
Flu tests are turning positive more often
CDC published its Week 36 report on September 18, covering the week ending September 12. Clinical laboratories tested 28,999 specimens and found influenza in 514 of them, a positivity rate of 1.8%. Public health laboratories confirmed 64 positives and subtyped 49: 44 were influenza A(H1N1)pdm09 and five were A(H3N2). No specimen contained H5 avian influenza, and no new human infection with a novel influenza virus was reported. 1
Outpatient clinics recorded fever with a cough or sore throat in 1.6% of visits; among children ages 0 through 4 the share was 4.8%. Emergency departments logged a flu discharge diagnosis in 0.3% of visits. Hospital networks reported 1,486 flu admissions during the week, or 0.4 per 100,000 people, with the highest rate among adults 65 and older at 1.5. Laboratory-confirmed flu hospitalizations since October 1 reached 31,186. Fifty-five jurisdictions reported minimal respiratory activity, and none reported low, moderate, high, or very high. 1
One influenza-associated pediatric death was reported during the week. The death occurred in February, during Week 7 of the 2025–26 season, and brings that season's reported total to 193 children. Late reports arrive for months after a season ends, so a February death appearing in a September report is a backfill of last season's total rather than a new event. 1
What this season's shot can do, and the number that is still missing
CDC has not published a vaccine effectiveness estimate for the 2026–27 season. Its vaccination-trends page, reported on September 18, still carries 2025–26 data through May 23 and states that weekly updates for that season ended May 29. 4 The agency's effectiveness index lists seasons through 2025–26, both marked preliminary. 5 A season's first estimate follows its cases, and the past two seasons' first numbers arrived in February and March.
Last season's figures are the most recent real numbers available, and they set a fair expectation. In CDC's largest network, which covers outpatient and emergency-department visits, vaccination lowered the odds of testing positive for flu by 36% across all ages, with a confidence interval of 33% to 39%. A second network estimated 24% (8% to 38%). For children 6 months through 17 years, estimates were 38% (33% to 43%) in the large network and 41% (25% to 53%) in a pediatric network. For adults 65 and older, the estimate was 30% (24% to 36%). Hospital-based estimates were close to those: 31% (23% to 39%) across all ages and 31% (21% to 39%) for adults 65 and older. 6
A figure such as 36% describes partial protection. In those studies, vaccinated people were about a third less likely to test positive for flu when they sought care for a respiratory illness. Effectiveness moves from season to season: the same CDC table runs from 19% in 2014–15 to 60% in 2010–11, with 2025–26 at 36%. 5
What is inside this season's shot
Every flu vaccine sold in the United States this season is trivalent, covering one influenza A(H1N1) virus, one influenza A(H3N2) virus, and one influenza B/Victoria virus. 3 All three components changed from last season's, including the H3N2 component, which was updated to target the subclade K virus that spread widely during 2025–26. 7
For a household weighing whether to wait for a newer formulation, the updated formulation is the one being shipped now, and the FDA expects an adequate and diverse supply for the season. 7
Measles: 3,471 cases, with a slowing weekly curve
CDC updated its measles page on September 18 with figures reported through September 17 at noon. The United States has recorded 3,471 confirmed cases in 2026, reported by 47 jurisdictions plus 17 cases among international visitors. The year has produced 39 outbreaks, and 95% of cases, or 3,296, are outbreak-associated. 2
Hospitalizations reached 301, equal to 9% of confirmed cases and up from 8% a week earlier. Among children under 5, 13% of cases were hospitalized; among adults 20 and older, 12%. Where vaccination status was known, 95% of patients were unvaccinated or had no documented doses. 2
Cases remain concentrated in a handful of states: Pennsylvania has reported 745, South Carolina 670, Utah 527, Texas 192, and Virginia 178. The weekly count of new cases by rash onset has fallen through the most recent complete weeks — 216 in the week starting August 23, 158 the following week, and 132 in the week starting September 6. The week starting September 13 shows 24 cases so far, in a count that is still filling in. 2
The page lists no confirmed measles deaths for 2026. It states that the National Center for Health Statistics has no 2026 death records indicating measles as the underlying cause, and that CDC is working with the Council of State and Territorial Epidemiologists on a standard case definition for measles deaths so jurisdictions classify them the same way. 2 Two doses of MMR are 97% effective at preventing measles, and kindergarten coverage stood at 92.4% for the 2025–26 school year. 2
The pipeline, and one study worth knowing about
Moderna's mFLUSIVA remains approved for adults 50 and older, with the 65-and-older indication granted under accelerated approval based on immune responses. The company's August 5 announcement expects availability during the 2026–27 respiratory season and offers no local stocking, price, or coverage information. 8
Pfizer's COVID-19 update archive shows no respiratory-vaccine entry since August 27, when the FDA approved the XFG-adapted COMIRNATY formula for adults 65 and older and for people ages 5 through 64 with a condition that raises their risk of severe COVID-19. 9
Immunize.org posted its 2026–27 influenza standing orders on September 16 — the documents that let a clinic or pharmacy vaccinate an adult or a child without an individual prescription — drawn from CDC's season guidance and the recommendations of the AAP, AAFP, ACOG, and IDSA. 10 CDC's routine immunization schedule pages still carry the July 2, 2025 addendum, and the week brought no change to the childhood or adult schedule. 11
This week's clinically relevant study appeared in Vaccine on September 17. A randomized trial in Thailand compared a high-dose (60 µg) with a standard-dose (15 µg) quadrivalent flu vaccine in 110 adults living with HIV. Seroprotection against all four strains was higher with the high dose, 61.5% against 39.2%, and the gap was widest for the A/H3N2 strain. Side effects were reported at similar rates in both groups, most often soreness at the injection site, with no severe vaccine-related events. The trial measured antibody levels one month after vaccination rather than illness outcomes, and 103 participants were included in the analysis. 12
Where does that leave a household? CDC's guidance already allows high-dose or adjuvanted flu vaccine as an acceptable option for some younger adults with weakened immune systems, including solid-organ transplant recipients on immunosuppressive medication, and leaves the choice to the clinician. 3 This trial adds to that conversation for people living with HIV; the standing guidance reads the same as it did last week.
What to do this week
- Book flu shots for everyone 6 months and older. September and October suit most people, with vaccination ideally finished by the end of October. Protection takes about two weeks to develop. 3
- Start young children who need two doses now. Children who require two doses should receive the first one as soon as vaccine is available, because the second comes at least four weeks later. 3
- Ask for a preferred formulation if someone in the house is 65 or older. Fluzone High-Dose, Flublok Recombinant, and Fluad Adjuvanted are the three CDC recommends preferentially for this age group. If none is in stock, any age-appropriate flu vaccine will do. 3
- Find the MMR records for school-age children. Two documented doses give about 97% protection. A pediatrician, state immunization registry, or county health department can retrieve records that have gone missing. 2
- For an immunocompromised household member, ask the clinician about a high-dose or adjuvanted flu vaccine. This applies to people living with HIV, transplant recipients on immunosuppressive medication, and others whose immune response to a standard shot may be weaker. 12
This brief provides general health journalism for U.S. families. A physician, pharmacist, or local public health department can evaluate individual medical histories and confirm clinic-specific product availability.
References
- 1
- 2Measles Cases and Outbreaks
cdc.gov
- 3
- 4
- 5
- 6
- 72026-2027 Flu Season
cdc.gov
- 8IR Insights: Introducing mFLUSIVA
modernatx.com
- 9
- 10IZ Express, September 16, 2026
immunize.org
- 11
- 12
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