Effectiveness Against Different Flu Viruses

Overview

There are many other viruses besides influenza viruses that can result in flu-like illness* (also known as influenza-like illness or "ILI") that spread during the flu season. These non-influenza viruses include, but are not limited to, the following:

• SARS-CoV-2 (the virus that causes COVID-19),
• rhinovirus (one cause of the "common cold"), and
• respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children and a major cause of severe respiratory illness in adults 65 years and older.

Types and subtypes

The amount of protection provided by a flu vaccine may vary by influenza virus type or subtype and by vaccine, even when the viruses used to make flu vaccines are similar to the influenza viruses that are causing illness that season. Since 2009, VE studies looking at how well flu vaccines protect against medically attended illnesses have suggested better protection against influenza B and influenza A(H1N1) viruses than against influenza A(H3N2) viruses.

A metanalysis of 76 published VE studies conducted in countries in both the Northern and Southern Hemispheres after the 2009-2010 flu pandemic found flu vaccines performed the best against influenza A(H1N1)pdm09 viruses followed by influenza B viruses and were least effective against influenza A(H3N2) viruses.

Across all the studies included in the meta-analysis, the pooled VE estimate against all influenza viruses for the Northern Hemisphere was 37 percent. For influenza A(H1N1)pdm09 viruses, the pooled VE estimate for the Northern Hemisphere was 56 percent. For influenza A(H3N2) viruses, the pooled VE estimate for the Northern Hemisphere was 22 percent.

And lastly, for influenza B viruses, the pooled VE estimate for studies from the Northern Hemisphere was 42 percent. Overall, VE estimates were lower when the viruses used to make flu vaccines were less similar to the influenza viruses causing illness that season and decreased with older age.

A(H3N2) viruses

There are several reasons why flu vaccine effectiveness against influenza A(H3N2) viruses may be lower compared to other influenza viruses.

1. While all influenza viruses undergo frequent genetic changes over time, the changes that have occurred in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses (i.e., antigenic changes) compared with influenza A(H1N1) and influenza B viruses. That means that between the time when influenza viruses are selected to begin producing vaccines and when flu vaccines are delivered, A(H3N2) viruses are more likely than A(H1N1) or influenza B viruses to have changed in ways that could impact how well the flu vaccines work.
2. Most inactivated seasonal flu vaccines produced by growing viruses in eggs. While all influenza viruses undergo changes when they are grown in eggs, changes in influenza A(H3N2) viruses tend to be more likely to result in antigenic changes compared with changes in other influenza viruses. These so-called "egg-adapted changes" are present in vaccine viruses recommended for use in vaccine production and may reduce their potential effectiveness against circulating influenza viruses. Other types of flu vaccines, such as cell-based and recombinant flu vaccines, are produce without the use of eggs, so the viruses used to make the vaccines do not undergo these changes. CDC is also using advanced molecular detection (AMD) techniques to improve flu vaccines by providing experts with more data to support the selection of candidate vaccine viruses.