Public Health Interventions and Overdose-Related Outcomes Among Persons With Opioid Use Disorder.

Importance: Given the high number of opioid overdose deaths in the US and the complex epidemiology of opioid use disorder (OUD), systems models can serve as a tool to identify opportunities for public health interventions. Objective: To estimate the projected 3-year association between public health interventions and opioid overdose-related outcomes among persons with OUD. Design, Setting, and Participants: This decision analytical model used a simulation model of the estimated US population aged 12 years and older with OUD that was developed and analyzed between January 2019 and December 2023. The model was parameterized and calibrated using 2019 to 2020 data and used to estimate the relative change in outcomes associated with simulated public health interventions implemented between 2021 and 2023. Main Outcomes and Measures: Projected OUD and medications for OUD (MOUD) prevalence in 2023 and number of nonfatal and fatal opioid-involved overdoses among persons with OUD between 2021 and 2023. Results: In a baseline scenario assuming parameters calibrated using 2019 to 2020 data remained constant, the model projected more than 16 million persons with OUD not receiving MOUD treatment and nearly 1.7 million persons receiving MOUD treatment in 2023. Additionally, the model projected over 5 million nonfatal and over 145 000 fatal opioid-involved overdoses among persons with OUD between 2021 and 2023. When simulating combinations of interventions that involved reducing overdose rates by 50%, the model projected decreases of up to 35.2% in nonfatal and 36.6% in fatal opioid-involved overdoses among persons with OUD. Interventions specific to persons with OUD not currently receiving MOUD treatment demonstrated the greatest reduction in numbers of nonfatal and fatal overdoses. Combinations of interventions that increased MOUD initiation and decreased OUD recurrence were projected to reduce OUD prevalence by up to 23.4%, increase MOUD prevalence by up to 137.1%, and reduce nonfatal and fatal opioid-involved overdoses among persons with OUD by 6.7% and 3.5%, respectively. Conclusions and Relevance: In this decision analytical model study of persons with OUD, findings suggested that expansion of evidence-based interventions that directly reduce the risk of overdose fatality among persons with OUD, such as through harm reduction efforts, could engender the highest reductions in fatal overdoses in the short-term. Interventions aimed at increasing MOUD initiation and retention of persons in treatment projected considerable improvement in MOUD and OUD prevalence but could require a longer time horizon for substantial reductions in opioid-involved overdoses.

Characterization of Nonfatal Opioid, Cocaine, Methamphetamine, and Polydrug Exposure and Clinical Presentations Reported to the Toxicology Investigators Consortium Core Registry, January 2010-December 2021.

INTRODUCTION: To characterize and compare opioid-only, cocaine-only, methamphetamine-only, opioid-and-cocaine exposure, and opioid-and-methamphetamine exposure and to examine clinical presentations, leading to a better understanding of overdose effects involving these drug exposures. METHODS: We examined drug exposures in the Toxicology Investigators Consortium (ToxIC) Core Registry from January 2010 to December 2021, a case registry of patients presenting to participating healthcare sites that receive a medical toxicology consultation. Demographic and clinical presentations of opioid-only, cocaine-only, methamphetamine-only, and opioid-and-cocaine exposure, and opioid-and-methamphetamine exposure consultations were described; differences between single and polydrug exposure subgroups were calculated to determine statistical significance. Clinical presentations associated with exposures were evaluated through calculated adjusted relative risk. RESULTS: A total of 3,883 consultations involved opioids, cocaine, methamphetamine, opioid-and-cocaine exposure, or opioid-and-methamphetamine exposure. Opioid-only (n = 2,268, 58.4%) and methamphetamine-only (n = 712, 18.3%) comprised most consultations. There were significant differences in clinical presentations between exposure subgroups. Opioid-and-cocaine exposure consultations were 8.15 times as likely to present with a sympathomimetic toxidrome than opioid-only. Conversely, opioid-and-cocaine exposure and opioid-and-methamphetamine exposure were 0.32 and 0.42 times as likely to present with a sympathomimetic toxidrome compared to cocaine-only and methamphetamine-only consultations, respectively. Opioid-and-cocaine exposure was 0.67 and opioid-and-methamphetamine exposure was 0.74 times as likely to present with respiratory depression compared to opioid-only consultations. Similarly, opioid-and-cocaine exposure was 0.71 and opioid-and-methamphetamine exposure was 0.78 times as likely to present with CNS depression compared to opioid-only consultations. CONCLUSIONS: Used in combination, opioids and stimulants may mask typical clinical presentations of one another, misattributing incorrect drugs to overdose in both clinical treatment and public health surveillance.

Analysis of trends and usage of ICD-10-CM discharge diagnosis codes for poisonings by fentanyl, tramadol, and other synthetic narcotics in emergency department data.

Synthetic opioids, including illicitly manufactured fentanyls, are driving recent increases in US overdose deaths. Beginning October 2020, the International Classification of Diseases, 10th revision, Clinical Modification (ICD-10-CM) code for poisonings involving synthetic narcotics (T40.4X) was split into three codes: fentanyl (T40.41), tramadol (T40.42), and other synthetic narcotics (T40.49). Emergency department data from October 2019-September 2021 in the Centers for Disease Control and Prevention's National Syndromic Surveillance Program BioSense platform were queried for synthetic opioid codes in the chief complaint and discharge diagnosis fields. Trend analyses assessed average monthly percent change overall and by sex and age. Emergency department visits for overdoses involving synthetic narcotics increased on average 3.2 % each month before the code split and 4.8 % after. Visits with fentanyl codes drove this increase after the split, accounting for most visits among males, females, and every age group except ≥ 65 years. The average monthly percent increase for ED visits for fentanyl-involved overdoses was greater than for all synthetic narcotics combined (i.e., T40.41, T40.42, and/or T40.49), suggesting that the old code (T40.4X) masked the full extent of the increase in ED visits for fentanyl overdoses. Usage of these new codes can improve tracking of non-fatal synthetic opioid overdose trends.

Trends and correlates of cocaine use among adults in the United States, 2006-2019.

BACKGROUND: Cocaine is the most commonly reported illicit stimulant used in the U.S., yet limited research has examined recent changes in cocaine use patterns and co-occurring substance use and mental health characteristics among adults using cocaine. METHODS: Self-report data from adults (age 18 years or older) participating in the 2006 to 2019 National Surveys on Drug Use and Health (NSDUH) were used to estimate trends in prevalence of past-year cocaine use by demographic characteristics, cocaine use disorder, cocaine injection, frequency of use. For 2018-2019, prevalence of co-occurring past-year use of other illicit and prescription substances and mental health characteristics were estimated. Multivariable logistic regression examined demographic, substance use, and mental health characteristics associated with past-year cocaine use in 2018-2019. RESULTS: The annual average estimated prevalence of past-year cocaine use among adults was highest in 2006-2007 (2.51%), declined to 1.72% in 2010-2011, and then increased to 2.14% in 2018-2019. The annual average estimated prevalence of past-year cocaine use disorder was highest in 2006-2007 (0.71%) and declined to 0.37% in 2018-2019. Characteristics associated with higher adjusted odds of past-year cocaine use included: males; ages 18-49; Hispanic ethnicity; income <$20,000; large or small metro counties; use of other substances (nicotine, alcohol, marijuana, sedative/tranquilizers, prescription opioids, prescription stimulants, heroin, and methamphetamine); and serious psychological distress and suicidal ideation or attempt. CONCLUSION: Additional efforts to support prevention and response capacity in communities, expand linkages to care and retention for substance use and mental health, and enhance collaborations between public health and public safety are needed.

Development and Validation of a Syndrome Definition to Identify Suspected Nonfatal Heroin-Involved Overdoses Treated in Emergency Departments.

CONTEXT: The Centers for Disease Control and Prevention (CDC) works closely with states and local jurisdictions that are leveraging data from syndromic surveillance systems to identify meaningful changes in overdose trends. CDC developed a suspected nonfatal heroin overdose syndrome definition for use with emergency department (ED) data to help monitor trends at the national, state, and local levels. OBJECTIVE: This study assesses the percentage of true-positive unintentional and undetermined intent heroin-involved overdose (UUHOD) captured by this definition. DESIGN/SETTING: CDC applied the UUHOD definition to ED data available in CDC's National Syndromic Surveillance Program (NSSP). Data were analyzed from 18 states that shared access to their syndromic data in NSSP with the CDC overdose morbidity team. Data were analyzed using queries and manual reviews to identify heroin overdose diagnosis codes and text describing chief complaint reasons for ED visits. MEASURES: The percentage of true-positive UUHOD was calculated as the number of true-positives divided by the number of total visits captured by the syndrome definition. RESULTS: In total, 99 617 heroin overdose visits were identified by the syndrome definition. Among 95 323 visits identified as acute heroin-involved overdoses, based on reviews of chief complaint text and diagnosis codes, 967 (1.0%) were classified as possible intentional drug overdoses. Among all 99 617 visits, 94 356 (94.7%) were classified as true-positive UUHOD; 2226 (2.2%) and 3035 (3.0%) were classified as "no" and "maybe" UUHOD, respectively. CONCLUSION: Analysis of the CDC heroin overdose syndrome definition determined that nearly all visits were captured accurately for patients presenting to the ED for a suspected acute UUHOD. This definition will continue to be valuable for ongoing heroin overdose surveillance and epidemiologic analysis of heroin overdose patterns. CDC will evaluate possible definition refinements as new products and terms for heroin overdose emerge.

Suspected Nonfatal Drug-Related Overdoses Among Youth in the US: 2016-2019.

BACKGROUND AND OBJECTIVES: During the current drug overdose crisis, the United States is experiencing a significant number of overdose deaths, hospitalizations, and emergency department visits. Given the vulnerability of young persons to substance use, it is important to assess how this crisis affects the nation's youth. In this study, we investigate trends in suspected nonfatal drug-related overdoses (all-drugs, opioids, heroin, and stimulants) among youth using syndromic surveillance data from 2016 to 2019. METHODS: A retrospective analysis of emergency department syndromic surveillance data were used to detect quarterly trends in suspected drug overdoses from April 2016 through September 2019 among youth aged 0 to 10, 11 to 14, and 15 to 24 years. Syndrome definitions were developed using chief complaint free-text and discharge diagnosis codes to identify overdoses involving all-drugs, opioids, heroin, and stimulants. Pearson χ2 tests detected quarter-to-quarter changes, and joinpoint regression analysis assessed trends over time. RESULTS: On average, there was a 2.0% increase for youth aged 0 to 10 years and a 2.3% increase for youth aged 11 to 14 years for suspected all-drug overdoses. Suspected heroin overdoses decreased by an average of 3.3% per quarter for youth aged 15 to 24 years. Among all age groups, suspected stimulant overdoses increased across the study period, 3.3% for 0 to 10-year-olds, 4.0% for 11- to 14-year-olds, and 2.3% for 15- to 24-year-olds. CONCLUSIONS: Suspected stimulant-involved drug overdoses appear to be rising among youth. These findings could inform targeted interventions, such as stimulant-focused prevention, and comprehensive approaches, including school-based prevention and other strategies to lower morbidity and mortality.

Nonfatal Drug Overdoses Treated in Emergency Departments - United States, 2016-2017.

In 2017, drug overdoses caused 70,237 deaths in the United States, a 9.6% rate increase from 2016 (1). Monitoring nonfatal drug overdoses treated in emergency departments (EDs) is also important to inform community prevention and response activities. Analysis of discharge data provides insights into the prevalence and trends of nonfatal drug overdoses, highlighting opportunities for public health action to prevent overdoses. Using discharge data from the Healthcare Cost and Utilization Project's (HCUP) Nationwide Emergency Department Sample (NEDS), CDC identified nonfatal overdoses for all drugs, all opioids, nonheroin opioids, heroin, benzodiazepines, and cocaine and examined changes from 2016 to 2017, stratified by drug type and by patient, facility, and visit characteristics. In 2017, the most recent year for which population-level estimates of nonfatal overdoses can be generated, a total of 967,615 nonfatal drug overdoses were treated in EDs, an increase of 4.3% from 2016, which included 305,623 opioid-involved overdoses, a 3.1% increase from 2016. From 2016 to 2017, the nonfatal overdose rates for all drug types increased significantly except for those involving benzodiazepines. These findings highlight the importance of continued surveillance of nonfatal drug overdoses treated in EDs to inform public health actions and, working collaboratively with clinical and public safety partners, to link patients to needed recovery and treatment resources (e.g., medication-assisted treatment).

Patterns and Characteristics of Methamphetamine Use Among Adults - United States, 2015-2018.

Methamphetamine is a highly addictive central nervous system stimulant. Methamphetamine use is associated with a range of health harms, including psychosis and other mental disorders, cardiovascular and renal dysfunction, infectious disease transmission, and overdose (1,2). Although overall population rates of methamphetamine use have remained relatively stable in recent years (3), methamphetamine availability and methamphetamine-related harms (e.g., methamphetamine involvement in overdose deaths and number of treatment admissions) have increased in the United States* (4,5); however, analyses examining methamphetamine use patterns and characteristics associated with its use are limited. This report uses data from the 2015-2018 National Surveys on Drug Use and Health (NSDUHs) to estimate methamphetamine use rates in the United States and to identify characteristics associated with past-year methamphetamine use. Rates (per 1,000 adults aged ≥18 years) for past-year methamphetamine use were estimated overall, by demographic group, and by state. Frequency of past-year use and prevalence of other substance use and mental illness among adults reporting past-year use were assessed. Multivariable logistic regression examined characteristics associated with past-year use. During 2015-2018, the estimated rate of past-year methamphetamine use among adults was 6.6 per 1,000. Among adults reporting past-year methamphetamine use, an estimated 27.3% reported using on ≥200 days, 52.9% had a methamphetamine use disorder, and 22.3% injected methamphetamine. Controlling for other factors, higher adjusted odds ratios for past-year use were found among men; persons aged 26-34, 35-49, and ≥50 years; and those with lower educational attainment, annual household income <$50,000, Medicaid only or no insurance, those living in small metro and nonmetro counties,† and those with co-occurring substance use and co-occurring mental illness. Additional efforts to build state and local prevention and response capacity, expand linkages to care, and enhance public health and public safety collaborations are needed to combat increasing methamphetamine harms.

Resurgent Methamphetamine Use at Treatment Admission in the United States, 2008-2017.

Objectives. To evaluate trends and correlates of methamphetamine use in the United States.Methods. Data are from 15 747 334 drug-related treatment admissions among persons aged 12 years or older in the 2008-2017 Treatment Episode Data Set. We analyzed trends and used multivariable logistic regression.Results. Methamphetamine-related admissions increased from 15.1% of drug-related treatment admissions in 2008 to 23.6% in 2017. Increases occurred among nearly all demographic groups. Methamphetamine injection increased from 17.5% of admissions in 2008 to 28.4% in 2017. Among methamphetamine-related admissions, heroin use increased from 5.3% of admissions in 2008 to 23.6% in 2017. Characteristics associated with increased odds of reporting methamphetamine use at admission included female sex; admissions aged 35 to 44 years; admissions in the Midwest, South, and West; unemployment; not in labor force; living dependent; living homeless; and having a referral from criminal justice, a health care provider, or other community treatment source.Conclusions. Treatment admissions involving methamphetamine use increased significantly over the past decade and appear to be linked to the ongoing opioid crisis in the United States. Efforts to mobilize public health prevention, treatment, and response strategies to address rising methamphetamine use and overdose are needed.

Human parainfluenza virus circulation, United States, 2011-2019.

BACKGROUND: Human parainfluenza viruses (HPIVs) cause upper and lower respiratory tract illnesses, most frequently among infants and young children, but also in the elderly. While seasonal patterns of HPIV types 1-3 have been described, less is known about national patterns of HPIV-4 circulation. OBJECTIVES: To describe patterns of HPIVs circulation in the United States (US). STUDY DESIGN: We used data from the National Respiratory and Enteric Virus Surveillance System (NREVSS), a voluntary passive laboratory-based surveillance system, to characterize the epidemiology and circulation patterns of HPIVs in the US during 2011-2019. We summarized the number of weekly aggregated HPIV detections nationally and by US census region, and used a subset of data submitted to NREVSS from public health laboratories and several clinical laboratories during 2015-2019 to analyze differences in patient demographics. RESULTS: During July 2011 - June 2019, 2,700,135 HPIV tests were reported; 122,852 (5 %) were positive for any HPIV including 22,446 for HPIV-1 (18 %), 17,474 for HPIV-2 (14 %), 67,649 for HPIV-3 (55 %), and 15,283 for HPIV-4 (13 %). HPIV testing increased substantially each year. The majority of detections occurred in children aged ≤ 2 years (36 %) with fluctuations in the distribution of age by type. CONCLUSIONS: HPIVs were detected year-round during 2011-2019, with type-specific year-to-year variations in circulation patterns. Among HPIV detections where age was known, the majority were aged ≤ 2 years. HPIV-4 exhibited an annual fall-winter seasonality, both nationally and regionally. Continued surveillance is needed to better understand national patterns of HPIV circulation.

Burden of influenza-associated respiratory hospitalizations in the Americas, 2010-2015.

BACKGROUND: Despite having influenza vaccination policies and programs, countries in the Americas underutilize seasonal influenza vaccine, in part because of insufficient evidence about severe influenza burden. We aimed to estimate the annual burden of influenza-associated respiratory hospitalizations in the Americas. METHODS: Thirty-five countries in the Americas with national influenza surveillance were invited to provide monthly laboratory data and hospital discharges for respiratory illness (International Classification of Diseases 10th edition J codes 0-99) during 2010-2015. In three age-strata (<5, 5-64, and ≥65 years), we estimated the influenza-associated hospitalizations rate by multiplying the monthly number of respiratory hospitalizations by the monthly proportion of influenza-positive samples and dividing by the census population. We used random effects meta-analyses to pool age-group specific rates and extrapolated to countries that did not contribute data, using pooled rates stratified by age group and country characteristics found to be associated with rates. RESULTS: Sixteen of 35 countries (46%) contributed primary data to the analyses, representing 79% of the America's population. The average pooled rate of influenza-associated respiratory hospitalization was 90/100,000 population (95% confidence interval 61-132) among children aged <5 years, 21/100,000 population (13-32) among persons aged 5-64 years, and 141/100,000 population (95-211) among persons aged ≥65 years. We estimated the average annual number of influenza-associated respiratory hospitalizations in the Americas to be 772,000 (95% credible interval 716,000-829,000). CONCLUSIONS: Influenza-associated respiratory hospitalizations impose a heavy burden on health systems in the Americas. Countries in the Americas should use this information to justify investments in seasonal influenza vaccination-especially among young children and the elderly.

Birth Cohort Effects in Influenza Surveillance Data: Evidence That First Influenza Infection Affects Later Influenza-Associated Illness.

BACKGROUND: The evolution of influenza A viruses results in birth cohorts that have different initial influenza virus exposures. Historically, A/H3 predominant seasons have been associated with more severe influenza-associated disease; however, since the 2009 pandemic, there are suggestions that some birth cohorts experience more severe illness in A/H1 predominant seasons. METHODS: United States influenza virologic, hospitalization, and mortality surveillance data during 2000-2017 were analyzed for cohorts born between 1918 and 1989 that likely had different initial influenza virus exposures based on viruses circulating during early childhood. Relative risk/rate during H3 compared with H1 predominant seasons during prepandemic versus pandemic and later periods were calculated for each cohort. RESULTS: During the prepandemic period, all cohorts had more influenza-associated disease during H3 predominant seasons than H1 predominant seasons. During the pandemic and later period, 4 cohorts had higher hospitalization and mortality rates during H1 predominant seasons than H3 predominant seasons. CONCLUSIONS: Birth cohort differences in risk of influenza-associated disease by influenza A virus subtype can be seen in US influenza surveillance data and differ between prepandemic and pandemic and later periods. As the population ages, the amount of influenza-associated disease may be greater in future H1 predominant seasons than H3 predominant seasons.

Update: Influenza Activity - United States and Worldwide, May 20-October 13, 2018.

During May 20-October 13, 2018,* low levels of influenza activity were reported in the United States, with a mix of influenza A and B viruses circulating. Seasonal influenza activity in the Southern Hemisphere was low overall, with influenza A(H1N1)pdm09 predominating in many regions. Antigenic testing of available influenza A and B viruses indicated that no significant antigenic drift in circulating viruses had emerged. In late September, the components for the 2019 Southern Hemisphere influenza vaccine were selected and included an incremental update to the A(H3N2) vaccine virus used in egg-based vaccine manufacturing; no change was recommended for the A(H3N2) component of cell-manufactured or recombinant influenza vaccines. Annual influenza vaccination is the best method for preventing influenza illness and its complications, and all persons aged ≥6 months who do not have contraindications should receive influenza vaccine, preferably before the onset of influenza circulation in their community, which often begins in October and peaks during December-February. Health care providers should offer vaccination by the end of October and should continue to recommend and administer influenza vaccine to previously unvaccinated patients throughout the 2018-19 influenza season (1). In addition, during May 20-October 13, a small number of nonhuman influenza "variant" virus infections† were reported in the United States; most were associated with exposure to swine. Although limited human-to-human transmission might have occurred in one instance, no ongoing community transmission was identified. Vulnerable populations, especially young children and other persons at high risk for serious influenza complications, should avoid swine barns at agricultural fairs, or close contact with swine.§.

Update: Influenza Activity in the United States During the 2017-18 Season and Composition of the 2018-19 Influenza Vaccine.

The United States 2017-18 influenza season (October 1, 2017-May 19, 2018) was a high severity season with high levels of outpatient clinic and emergency department visits for influenza-like illness (ILI), high influenza-related hospitalization rates, and elevated and geographically widespread influenza activity across the country for an extended period. Nationally, ILI activity began increasing in November, reaching an extended period of high activity during January-February, and remaining elevated through March. Influenza A(H3N2) viruses predominated through February and were predominant overall for the season; influenza B viruses predominated from March onward. This report summarizes U.S. influenza activity* during October 1, 2017-May 19, 2018.†.

Mapping of the US Domestic Influenza Virologic Surveillance Landscape.

Influenza virologic surveillance is critical each season for tracking influenza circulation, following trends in antiviral drug resistance, detecting novel influenza infections in humans, and selecting viruses for use in annual seasonal vaccine production. We developed a framework and process map for characterizing the landscape of US influenza virologic surveillance into 5 tiers of influenza testing: outpatient settings (tier 1), inpatient settings and commercial laboratories (tier 2), state public health laboratories (tier 3), National Influenza Reference Center laboratories (tier 4), and Centers for Disease Control and Prevention laboratories (tier 5). During the 2015-16 season, the numbers of influenza tests directly contributing to virologic surveillance were 804,000 in tiers 1 and 2; 78,000 in tier 3; 2,800 in tier 4; and 3,400 in tier 5. With the release of the 2017 US Pandemic Influenza Plan, the proposed framework will support public health officials in modeling, surveillance, and pandemic planning and response.

Human coronavirus circulation in the United States 2014-2017.

BACKGROUND: Human coronaviruses (HCoVs) -OC43, -229E, -NL63 and -HKU1 cause upper and lower respiratory tract infections. HCoVs are globally distributed and the predominant species may vary by region or year. Prior studies have shown seasonal patterns of HCoV species and annual variation in species prevalence but national circulation patterns in the US have not yet been described. OBJECTIVES: To describe circulation patterns of HCoVs -OC43, -229E, -NL63 and -HKU1 in the US. STUDY DESIGN: We reviewed real-time reverse transcription polymerase chain reaction (rRT-PCR) test results for HCoV-OC43, -229E, -NL63 and -HKU1 reported to The National Respiratory and Enteric Virus Surveillance System (NREVSS) by U.S. laboratories from July 2014-June 2017. We calculated the total number of tests and percent positive by week. For a subset of HCoV positive submissions with age and sex of the patient available, we tested for differences in age and sex across the four HCoV species using Chi Square and Kruskal Wallace tests. RESULTS: 117 laboratories reported 854,575 HCoV tests; 2.2% were positive for HCoV-OC43, 1.0% for HCoV-NL63, 0.8% for HCoV-229E, and 0.6% for HCoV-HKU1. The percentage of positive tests peaked during December - March each year. No significant differences in sex were seen across species, although a significant difference in age distribution was noted. CONCLUSIONS: Common HCoVs may have annual peaks of circulation in winter months in the US, and individual HCoVs may show variable circulation from year to year. Different HCoV species may be detected more frequently in different age groups. Further years of data are needed to better understand patterns of activity for HCoVs.

Update: Influenza Activity - United States, October 1, 2017-February 3, 2018.

Influenza activity in the United States began to increase in early November 2017 and rose sharply from December through February 3, 2018; elevated influenza activity is expected to continue for several more weeks. Influenza A viruses have been most commonly identified, with influenza A(H3N2) viruses predominating, but influenza A(H1N1)pdm09 and influenza B viruses were also reported. This report summarizes U.S. influenza activity* during October 1, 2017-February 3, 2018,† and updates the previous summary (1).

Update: Influenza Activity - United States, October 1-November 25, 2017.

Influenza activity in the United States was low during October 2017, but has been increasing since the beginning of November. Influenza A viruses have been most commonly identified, with influenza A(H3N2) viruses predominating. Several influenza activity indicators were higher than is typically seen for this time of year. The majority of influenza viruses characterized during this period were genetically or antigenically similar to the 2017-18 Northern Hemisphere cell-grown vaccine reference viruses. These data indicate that currently circulating viruses have not undergone significant antigenic drift; however, circulating A(H3N2) viruses are antigenically less similar to egg-grown A(H3N2) viruses used for producing the majority of influenza vaccines in the United States. It is difficult to predict which influenza viruses will predominate in the 2017-18 influenza season; however, in recent past seasons in which A(H3N2) viruses predominated, hospitalizations and deaths were more common, and the effectiveness of the vaccine was lower. Annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. Multiple influenza vaccines are approved and recommended for use during the 2017-18 season, and vaccination should continue to be offered as long as influenza viruses are circulating and unexpired vaccine is available. This report summarizes U.S. influenza activity* during October 1-November 25, 2017 (surveillance weeks 40-47).†.