Influenza Activity Still Low in United States

Troy Brown, RN

November 20, 2018

Small increases in influenza activity occurred in the United States during the week ending November 10 (week 45), but activity is still low, according to the US Centers for Disease Control and Prevention (CDC).

At 1.9%, the nationwide percentage of patient visits reported through the US Outpatient Influenza-like Illness Surveillance Network for influenza-like illness (ILI) remained below the national baseline of 2.2%. They define ILI as having a body temperature of 100 °F (37.8 °C) or greater and cough and/or sore throat.

The proportion of deaths resulting from pneumonia and influenza (P&I) was lower than the system-specific epidemic threshold in the National Center for Health Statistics (NCHS) Mortality Surveillance System. NCHS mortality data published November 15 showed that 5.4% of the deaths that occurred during week 45 resulted from P&I — "below the epidemic threshold of 6.2% for week 44" — the CDC notes in its weekly US Influenza Surveillance Report for the week ending November 10.

No child deaths related to influenza were reported during week 45.

Most States Report Little or Minimal ILI Activity

Georgia experienced moderate activity that was at least as high as its region-specific baseline level. Activity was moderate in Georgia during week 44 as well.

Low activity was reported in Alabama, Louisiana, South Carolina, Utah, and Virginia during week 45; activity was low in Alabama, Louisiana, and Missouri during week 44.

The District of Columbia, Puerto Rico, and 44 states reported minimal ILI activity during week 45.

Regional Influenza Activity in Kentucky, Maryland, and Oregon

In terms of geographic spread, Kentucky, Maryland, and Oregon experienced regional influenza activity. Regional outbreaks are those with influenza cases or increases in ILI and recent laboratory-confirmed cases of influenza in at least two but less than half the regions of the state in which there is recent laboratory evidence of influenza, according to the latest report from the Kentucky Department for Public Health.

For example, Kentucky had 12 laboratory-confirmed cases of influenza, for a total of 78 this season. No patients died during week 45. Two adults and no children have died this season in the state. There have been no outbreaks in long-term care facilities this season. Influenza has been confirmed in 11 of 11 regions, and 16 of 17 regions have had ILI activity this influenza season.

Maryland has reported minimal activity since the week ending October 6 (week 40) and reported regional activity for the third week in a row. Regional activity is an indicator of where infections are occurring, not how severe cases are, the authors state.

In Oregon, during week 45, emergency department visits for ILI increased slightly to 1.1% from 1.0% the week before. Outbreaks of ILI were not reported during week 45; one occurred during week 44. No children died during either week.

Guam and 10 states (Arizona, Connecticut, Georgia, Idaho, Louisiana, Massachusetts, New Hampshire, New Jersey, North Dakota, and Texas) reported local activity, meaning there have been influenza outbreaks or increases in ILI cases and recent laboratory-confirmed influenza cases in a single region of the state.

The District of Columbia, Puerto Rico, the US Virgin Islands, and 35 states reported sporadic activity. For example, there have been small numbers of laboratory-confirmed influenza cases or a single laboratory-confirmed influenza outbreak, but cases of ILI have not increased.

Mississippi and Virginia reported no influenza activity.

Influenza A Viruses Continue to Predominate

Influenza A(H1N1)pdm09 viruses have been most commonly reported by public health laboratories since late September 2018, although influenza A(H3N2) and influenza B viruses are also circulating.

Since early July, influenza A viruses continued to have been predominate in the United States; that trend continued during week 45. A low percentage of respiratory specimens tested positive for influenza in clinical laboratories.

Most influenza viruses that have been antigenically and genetically characterized are similar to reference viruses grown in cells that represented the 2018 – 2019 Northern Hemisphere influenza vaccine viruses.

All viruses tested since late May have been susceptible to the antiviral medications oseltamivir (Tamiflu, Roche), zanamivir (Relenza, GlaxoSmithKline), and peramivir (Rapivab, BioCryst).

Volume of Information May Make Parents Question Flu Vaccine

A nationally representative Web-based poll of 1977 parents found that 4 in 10 make their immunization decisions on the basis of information obtained not from their child's physician or healthcare provider but from other sources.

The CDC recommends yearly influenza vaccination for every child aged 6 months and older, yet almost one third of parents do not plan to have their child vaccinated this season, the Mott Report authors write.

"[T]here may be somewhat of an echo chamber of information sources about flu vaccine for children. Parents who decided to get flu vaccine for their child reported hearing or seeing information about flu vaccine that is largely in favor of flu vaccine — in fact, these parents reported four times as many information sources that prompted them to want to get their child vaccinated," the authors write.

"The opposite was true for parents who decided that their child will not get flu vaccine: they reported seven times as many information sources that made them question or not want to have their child vaccinated."

The Web-based survey asked the parents about their vaccination habits. Four of 10 made their immunization decisions on the basis of information from family, close friends, and other parents; these parents were more likely to refuse vaccination for their child.

Those who got their vaccination information from their child's healthcare provider, nurses, and medical staff were more likely to desire flu vaccination for their child. One fifth of parents said their child's provider had not made a vaccine recommendation.

Vaccine Effectiveness Drops Off in Children?

A study published online November 12 in Lancet Respiratory Medicine found that the effectiveness of the influenza vaccine falls off 9 months after immunization.

Shuo Feng, PhD, World Health Organization Collaborating Center for Infectious Disease Epidemiology and Control, School of Public Health, the University of Hong Kong, China, and colleagues conducted their study over five influenza seasons.

From September 1, 2012, to August 31, 2017, 15,695 children were hospitalized with pneumonia. Of those, 2500 (15.9%) tested positive for influenza, and 13,195 (84.1%) tested negative. Of the vaccinated children, 159 (6.4%) tested positive for influenza, and 1445 (11%) tested negative.

Most children were immunized in December of each influenza season. "Pooled vaccine effectiveness declined over the following months, dropping from 79% for September through December, to 67% for January to April, to 43% for May to August. In a separate analysis, the investigators estimated that vaccine effectiveness dropped by 2 to 5 percentage points each month," the authors explain.

In a related commentary, Lien Anh Ha Do, PhD, from Murdoch Children's Research Institute, Melbourne, Australia, and Kim Mulholland, MBBS, FRACP, MD, Murdoch Children's Research Institute and the Department of Pediatrics, University of Melbourne, Australia, and the London School of Hygiene and Tropical Medicine, United Kingdom, said the results "have important policy implications."

"Health planners rely heavily on modelling studies to help predict the effect of various vaccination strategies. In the absence of data on the rate of decline in vaccine efficacy, modelling studies have to make assumptions, often ignoring the effect of waning immunity," they write.

"The findings of the present study will raise important questions about some of the conclusions of the published models, probably reducing the modelled beneficial effects. This uncertainty in predicted effect is especially true in tropical countries where seasonal influenza is more unpredictable, as this study in Hong Kong shows. Annual vaccination of whole populations is a large and expensive commitment. Models predicting vaccine effectiveness must use accurate data for key parameters. The study by Feng and colleagues represents an important contribution in this area," they write.

The authors have disclosed no relevant financial relationships.

Lancet Respir Med. Published online November 12, 2018. Abstract

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