2020-2021 Flu Season

Summary

What was the 2020-2021 flu season like?

Flu activity was unusually low throughout the 2020-2021 flu season both in the United States and globally, despite high levels of testing. During September 28, 2020–May 22, 2021, in the United States, 1,675 (0.2%) of 818,939 respiratory specimens tested by U.S. clinical laboratories were positive for an influenza virus. The low level of flu activity during this past season contributed to dramatically fewer flu illnesses, hospitalizations, and deaths compared with previous flu seasons. For comparison, during the last three seasons before the pandemic, the proportion of respiratory specimens testing positive for influenza peaked between 26.2% and 30.3%. In terms of hospitalizations, the cumulative rate of laboratory-confirmed influenza-associated hospitalizations in the 2020-2021 season was the lowest recorded since this type of data collection began in 2005. For pediatric deaths, CDC received one report of a pediatric flu death in a child during the 2020–2021 flu season. Since flu deaths in children became nationally notifiable in 2004, reported flu deaths in children had previously ranged from a low of 37 (during 2011-2012) to a high of 199 (during 2019-2020).

What are possible explanations for the unusually low flu activity?

COVID-19 mitigation measures such as wearing face masks, staying home, hand washing, school closures, reduced travel, increased ventilation of indoor spaces, and physical distancing, likely contributed to the decline in 2020-2021 flu incidence, hospitalizations and deaths. Influenza vaccination may also have contributed to reduced flu illness during the 2020–2021 season. Flu vaccine effectiveness estimates for 2020-2021 are not available, but a record number of influenza vaccine doses (193.8 million doses) were distributed in the U.S. during 2020-2021.

How many people got vaccinated against flu during the 2020-2021 flu season and how does that compare to previous seasons?

CDC works each year to increase the number of people who receive a flu vaccine and eliminate barriers to vaccination. Influenza vaccine production and distribution in the US are primarily private sector endeavors, but during the 2020-2021 flu season, as part of efforts to maximize flu vaccination by increasing availability of vaccine, CDC purchased an additional 2 million doses of pediatric and 9.3 million doses of adult influenza vaccine to create a stockpile of vaccine in case of supply problems. While final estimates are pending, early estimates based on survey data suggest flu vaccination uptake for 2020-2021 was similar to the prior season, with small increases among some groups of people and small decreases among other groups of people. Preliminary estimates indicate that 50% to 55% of adults got a flu vaccine (compared with the 2019–2020 estimate of 48% by end of May 2020). Influenza vaccination coverage in children dropped 4.1 percentage points from 62.3% during 2019-2020 to 58.2% during 2020–2021 and estimates for pregnant people and health care personnel indicated slight decreases in influenza vaccine coverage. Racial and ethnic disparities in flu vaccine uptake persisted for children and adults. Because racial and ethnic minority groups might be at higher risk for developing serious illness, resulting from flu that may lead to hospitalization, flu vaccination is especially important for these communities.

Did new flu viruses circulate during the 2020-2021 flu season?

Flu viruses are constantly changing so it's not unusual for new flu viruses to appear each year. During the 2020-2021 flu season, there was very low circulation of seasonal flu viruses. During September 27, 2020–May 22, 2021 in the United States, 1,899 (0.2%) of 1,081,671 clinical samples tested were positive for an influenza virus (713 [37.5%] influenza A and 1,186 [62.5%] influenza B). During that same period, public health laboratories reported 61.4% of influenza positive samples were influenza A and 38.6% of positive samples were influenza B. The majority (52.5%) of influenza A viruses were H3N2, and the majority (60%) of influenza B viruses were of Victoria lineage.

In terms of novel influenza viruses, CDC reported five human infections with an influenza virus that usually spreads in pigs and not people (called a variant influenza virus) in the United States. All five of these infections occurred in people who reported that they had direct exposure to pigs or lived on a property where pigs were present. No person-to-person spread of variant influenza was identified associated with any of these patients. These types of infections occur in people rarely, and usually in the context of exposure to pigs, but are concerning because of their pandemic potential. Since 2005, a total of 489 variant influenza virus infections have been identified in the United States and reported to CDC.

Flu Vaccine

What flu viruses did the 2020-2021 flu vaccines protect against?

For 2020-2021, trivalent (three-component) egg-based vaccines contained:

• A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus (updated)
• A/Hong Kong/2671/2019 (H3N2)-like virus (updated)
• B/Washington/02/2019 (B/Victoria lineage)-like virus (updated)

Quadrivalent (four-component) egg-based vaccines, which protect against a second lineage of B viruses, contained:

• The three recommended viruses above, plus B/Phuket/3073/2013-like (Yamagata lineage) virus.

For 2020-2021, cell- or recombinant-based vaccines contained:

• A/Hawaii/70/2019 (H1N1)pdm09-like virus (updated)
• A/Hong Kong/45/2019 (H3N2)-like virus (updated)
• B/Washington/02/2019 (B/Victoria lineage)-like virus (updated)
• B/Phuket/3073/2013-like (Yamagata lineage) virus

Were there any changes to the 2020-2021 Northern Hemisphere vaccines from what was included in 2019-2020 U.S. flu vaccines?

Yes, 2020-2021 flu vaccines were updated to better match the flu viruses that were expected to circulate in the United States.

• The egg-based H1N1 vaccine component was updated from an A/Brisbane/02/2018 (H1N1)pdm09-like virus to an A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus.
• The cell- or recombinant-based H1N1 vaccine component was updated from an A/Brisbane/02/2018 (H1N1)pdm09-like virus to an A/Hawaii/70/2019 (H1N1)pdm09-like virus.
• The egg-based H3N2 vaccine component was updated from an A/Kansas/14/2017 (H3N2)-like virus to an A/Hong Kong/2671/2019 (H3N2)-like virus.
• The cell- or recombinant-based H3N2 vaccine component was updated from an A/Kansas/14/2017 (H3N2)-like virus to an A/Hong Kong/45/2019 (H3N2)-like virus.
• The B/Victoria lineage vaccine component was updated from a B/Colorado/06/2017 (B/Victoria lineage)-like virus to a B/Washington/02/2019 (B/Victoria lineage)-like virus.
• The B/Yamagata lineage vaccine component was not updated.

Were there any new vaccines licensed for use during the 2020-2021 flu season?

There were two new vaccines licensed for use during the 2020-2021 flu season.

• The first was a quadrivalent high-dose vaccine licensed for use in adults 65 years and older. This vaccine replaced the previously licensed trivalent high-dose vaccine.
• The second new vaccine for the 2020-2021 flu season was a quadrivalent adjuvanted vaccine licensed for use in adults 65 years and older.
• This vaccine was similar to the previously licensed trivalent vaccine containing MF59 adjuvant, but it has one additional influenza B component.

What flu vaccines were recommended during the 2020-2021 season?

For the 2020-2021 flu season, providers could choose to administer any licensed, age-appropriate flu vaccine (IIV, RIV4, or LAIV4) with no preference for any one vaccine over another.

Vaccine options included:

• Standard dose flu shots.
• High-dose shots for people 65 years and older.
• Shots made with adjuvant for people 65 years and older.
• Shots made with virus grown in cell culture. No eggs are involved in the production of this vaccine.
• Shots made using a vaccine production technology (recombinant vaccine) that do not require having a candidate vaccine virus (CVV) sample to produce.
• Live attenuated influenza vaccine(LAIV). – A vaccine made with attenuated (weakened) live virus that is given by nasal spray.

Flu Vaccine Availability

How many flu vaccines were available for the 2020-2021 flu season?

Flu vaccine is produced by private manufacturers, so supply depends on manufacturers. For the 2020-2021 season, manufacturers projected they would provide as many as 194-198 million doses of flu vaccine, which is more than the 175 million dose record set during the 2019-2020 flu season. 193.8 million doses of flu vaccine had been distributed in the United States as of February 26, 2021—the highest number of doses in a single flu season. CDC provided weekly updates on total flu vaccine doses distributed throughout the 2020-2021 flu season.

Were there delays in the availability of flu vaccine?

Influenza vaccine manufacturers did not report any significant delays in national flu vaccine supply or distribution during 2020-2021.

Were there enough doses of flu vaccine available for the 2020-2021 flu season?

Yes. Vaccine manufactures reported distributing 193.8 million doses of flu vaccine in the United States as of February 26, 2021. This was more flu vaccine than had ever previously been distributed in the United States. Some of this distributed vaccine was likely was not administered. In the United States in general, every year, there are a number of doses of flu vaccine that go unused.

Flu Vaccines During the COVID-19 Pandemic

Did we need to get a flu vaccine earlier during the 2020-2021 flu season (i.e. July/August)?

There was no change in CDC's recommendation on timing of vaccination last flu season. Getting vaccinated in July or August is too early, especially for older people, because of the likelihood of reduced protection against flu later in the flu season. September and October are good times to get vaccinated. However, as long as flu viruses are circulating, vaccination should continue, even in January or later.

Were there changes in how and where flu vaccines were given in fall and winter or 2020-2021?

Prior to the 2020-2021 flu season, CDC worked with health care providers and state and local health departments to develop contingency plans on how to vaccinate people against flu without increasing their risk of exposure to respiratory disease, like the virus that causes COVID-19. This included releasing Interim Guidance for Immunization Services During the COVID-19 Pandemic. Preliminary coverage data from September 2020 suggest there were some changes in where people got vaccinated early in 2020-2021. For example, the proportion of people reporting getting a flu vaccination at a store (53.8%) was significantly higher than the equivalent proportion for the 2019–20 season (34.9%), and the proportion reporting vaccination at a doctor's office was significantly lower than 2019–20 (29.7% vs 37.3%).

Were flu vaccines and COVID-19 vaccines given at the same time during the 2020-2021 season?

No. As recommended by ACIP during the 2020-2021 flu season, out of an abundance of caution, COVID-19 vaccines were administered alone, with a minimum interval of 14 days before or after administration of any other vaccines, including influenza vaccines.

What did CDC do to promote flu vaccination during the COVID-19 pandemic?

To address the importance of flu vaccination, especially during the COVID-19 pandemic, CDC increased the availability of vaccine, including purchasing an additional 2 million doses of pediatric flu vaccine and 9.3 million doses of adult flu vaccine to create a stockpile of vaccine in case of supply problems. CDC also emphasized the importance of flu vaccination for the entire flu season and conducted targeted communication outreach to specific groups who are at higher risk for complications from flu. These same groups are often at higher risk for COVID-19, too, so protecting them from influenza was important to decrease their risk of co-infection. Communication strategies for providers and the public included:

• Educational outreach activities by CDC, including social media, press conferences, web page spotlights, radio media tours, op-eds, and other publications,
• A digital campaign to educate the general public and people with who are at increased risk from influenza and COVID-19 complications,
• Special educational efforts to inform the general population, people with underlying health conditions, and African American and Hispanic audiences about the importance of flu vaccination, and
• Updated vaccination websites for the public and providers that highlight the safety precautions being implemented in health care facilities during the pandemic.

Flu Vaccine Coverage

What vaccine uptake estimates did CDC provide during the 2020-2021 season?

CDC developed a new Weekly National Influenza Vaccination Dashboard designed to share preliminary, in-season, weekly influenza vaccination coverage estimates and related data.

The dashboard included information on the number of influenza vaccine doses distributed in the United States, weekly flu vaccination coverage rates for children 6 months – 17 years old, monthly flu vaccination coverage rates among pregnant persons, and information on how many flu vaccines were administered in pharmacies and doctor's offices.

The data was updated weekly or monthly, depending on the data source, throughout the 2020-2021 influenza season; other data sources were added as they become available.

How did CDC track weekly flu vaccination coverage among children 6 months – 17 years old?

Influenza vaccination coverage among children was assessed through the National Immunization Survey-Flu (NIS-Flu), which provided weekly influenza vaccination coverage estimates for children 6 months–17 years old. NIS-Flu is a national random-digit-dialed cellular telephone survey of households conducted during the flu season (October-June).

How did CDC track monthly flu vaccination coverage among pregnant women?

Monthly flu vaccination coverage estimates among pregnant women are based on electronic health record (EHR) data from the Vaccine Safety Datalink (VSD), a collaboration between CDC's Immunization Safety Office and nine integrated health care organizations. Of note, because these estimates are based on data from nine integrated health care systems, they may not be representative of all pregnant women in the U.S.

How did CDC track the number of flu vaccines administered at pharmacies and doctor's offices?

CDC tracked the number of flu vaccines administered at pharmacies and doctor's offices by using new sources of vaccination data, including IQVIA data for vaccinations administered in retail pharmacies (e.g., chain, mass merchandise, food stores, and independent pharmacies) and doctor's offices.

When were the first flu vaccine uptake estimates provided the 2020-2021 season?

CDC launched the first weekly FluVaxView dashboard in December. The number of flu vaccine doses distributed, vaccination coverage estimates for children, and vaccinations administered in retail pharmacies and doctor's offices were updated weekly. Coverage estimates for pregnant women were updated monthly.

Was this the same kind of vaccine uptake information that has been provided in the past?

For each flu season since 2009-2010, CDC has estimated annual influenza vaccination coverage for the United States by using data from several nationally representative surveys: the Behavioral Risk Factor Surveillance System (BRFSS), the National Health Interview Survey (NHIS), and the National Immunization Survey-Flu (NIS-Flu). Internet panel surveys of adults, health care personnel, and pregnant women are also used.

CDC will continue to provide end of season estimates of influenza vaccination coverage from these data sources.

For the 2020-2021 flu season, CDC provided weekly updates on the number of flu vaccine doses distributed, vaccination coverage estimates for children, and the number of doses administered in pharmacies and doctor's offices. Coverage estimates for pregnant women were updated monthly.

Is CDC working to improve influenza vaccine uptake data?

CDC is exploring non-survey data sources, such as claims and other administrative data, to track flu vaccination coverage. For example, CDC is exploring ways to estimate within-season influenza vaccination coverage among adults using data on the number of doses administered in pharmacies and doctor's offices and estimates of the proportion of all influenza vaccinations that are received in these settings. CDC supports state and local jurisdictions in use of their immunization information systems to assess influenza vaccination coverage at the jurisdictional level.

Flu Surveillance Data Updates

Were there any updates in the methods for flu surveillance for 2020-2021?

For the 2020-2021 flu season, there were some changes to FluView surveillance methodology.

In addition to state-level data, the influenza-like-illness (ILI) activity map displayed ILI activity by Core-based Statistical Areas (CBSA), a U.S. geographic area defined by the Office of Management and Budget (OMB) that consists of one or more counties (or equivalents) anchored by an urban center of at least 10,000 people plus adjacent counties that are socioeconomically tied to the urban center by commuting.

Also, during most flu seasons, state and territorial health departments report the level of geographic spread of flu activity in their jurisdictions each week through the State and Territorial Epidemiologists Report. However, because COVID-19 and influenza have similar symptoms and it is difficult to differentiate the two without laboratory testing, reporting for this system was suspended for the 2020-2021 influenza season.

Finally, NCHS collects death certificate data for all deaths occurring in the United States, and these data are aggregated by the week of death occurrence. In previous flu seasons, the NCHS surveillance data were used to calculate the percent of all deaths occurring each week that had pneumonia and/or influenza (P&I) listed as a cause of death. Because many COVID-19-related deaths also have pneumonia, COVID-19 coded deaths were added to P&I to create the PIC (pneumonia, influenza, and/or COVID-19) classification. PIC includes all deaths with pneumonia, influenza, and/or COVID-19 listed on the death certificate.

Why was pneumonia, influenza, and COVID-19 (PIC) mortality data added to FluView Interactive?

CDC monitors flu deaths each week using death certificate data collected by the National Center for Health Statistics (NCHS). NCHS mortality surveillance data was used in previous years to calculate the percentage of all U.S. deaths occurring each week that had pneumonia and/or influenza (P&I) listed as a cause of death on the death certificate. Pneumonia is included because it is a frequent complication of severe influenza and increases in flu activity are associated with increases in pneumonia deaths. The weekly percentage of P&I deaths is compared to the expected percentage of deaths due to pneumonia to estimate the increase in pneumonia deaths, or excess deaths, due to influenza. Because pneumonia is also a frequent cause of death among people with COVID-19, COVID-19 coded deaths were added to P&I to create the PIC (pneumonia, influenza and/or COVID-19) mortality classification. CDC has displayed PIC mortality in its FluView report since week 40 of 2020. In addition, to make these data more easily downloadable and interactive, CDC incorporated PIC mortality data into its FluView Interactive data dashboard, an online data resource that accompanies the FluView report. Using FluView Interactive, users can download flu data and view this data via detailed, interactive graphs, charts, maps, and other visualizations.