During the 2020-2021 influenza season, fewer cases of influenza were reported than in any previous year since 1997, when data were first recorded.1FIGURE 12 shows the dramatic decline in the number of influenza-positive clinical samples reported to the Centers for Disease Control and Prevention (CDC) during the 2020-2021 influenza season compared with the 2019-2020 season. There was only one pediatric death attributed to influenza in 2020-2021, compared with a mean of 177 per year in the previous 3 seasons.
Deaths attributed to pneumonia and influenza were recorded over a recent 5-year period, with COVID-19 added in early mid-2020 (FIGURE 2).1 Total cumulative deaths for 2020-2021 were extremely high, mostly due to COVID-19. Of the relatively few influenza cases last season, 37.5% were caused by influenza A and 62.5% by influenza B. The extremely low incidence of influenza precludes determining influenza vaccine effectiveness for last season.1
In addition, other common respiratory pathogens—parainfluenza, adenoviruses, rhinoviruses, enteroviruses, and common coronaviruses—circulated last winter at historic lows.3 All of these historic lows can be attributed to the measures taken to mitigate the effect of the COVID-19 pandemic, including masks, social distancing, closure of certain venues that normally attract large crowds, and the closure of schools with a resulting increase in schooling at home. With the anticipated relaxation of these measures in 2021-2022, we can expect more influenza and other respiratory ailments due to common pathogens.
Updates to influenza vaccine recommendations
At its June 2021 meeting, the Advisory Committee on Immunization Practices (ACIP) approved the influenza vaccine recommendations for the 2021-2022 season.4 The central recommendation is unchanged: Everyone ≥ 6 months of age should receive a vaccine unless they have a contraindication. Updates to the previous recommendations include the content of the 2021 vaccines, the specific vaccines that will be available for different age groups, the timing of vaccine administration, advice on co-administration with COVID-19 vaccines, and the list of contraindications and precautions based on vaccine type.4
Viral composition of US vaccines for the 2021-22 season
The antigens that will be included in the 2021-2022 influenza vaccines are listed in TABLE 1.4 The B strains are the same as last year; the A strains have been updated. The H3N2 strain is the same in all vaccines, but the H1N1 strain differs based on whether the vaccine is egg based or non-egg based. The advantage of non-egg-based vaccines is that the production process does not take as long and can be delayed in an attempt to better reflect the influenza stains in worldwide circulation.
The influenza vaccines expected to be available for the 2021-22 season
TABLE 24 lists the influenza vaccines approved for use in the United States and the ages for which they are approved.4 All products for 2021-2022 will be quadrivalent, containing 2 type-A and 2 type-B antigens. The only change in age indications is that cell culture–based inactivated influenza vaccine (ccIIV4) (Flucelvax Quadrivalent) is now approved for use starting at age 2 years; previously it was approved starting at age 4 years.4
Timing of vaccination
The onsets and peaks of influenza disease occur at different times each year and can also vary by geographic location. An analysis of 36 influenza seasons starting in 1982 showed that peak activity occurred most frequently in February (15 seasons), followed by December (7 seasons), and January and March (6 seasons each).5 Only once did peak activity occur in October and once in November. This information, along with observational studies showing the waning of influenza vaccine effectiveness after 5 to 6 months, especially in the elderly, informed the ACIP decision to modify their recommendation on the timing of vaccination. The recommendation now states that vaccine should be administered by the end of October and that July and August would have been too early, especially for older adults.
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