FluMist, formally known as the Live Attenuated Influenza Vaccine (LAIV), is the needle-free influenza vaccine administered as a nasal spray. Unlike the traditional influenza shot, which contains inactivated virus components, FluMist uses a live, weakened form of the virus. Understanding the effectiveness of the nasal spray vaccine requires looking closely at its unique mechanism and the real-world data tracking its protective ability.
How the Nasal Spray Vaccine Works
The Live Attenuated Influenza Vaccine (LAIV) is fundamentally different from the standard flu shot because it contains live viruses that have been weakened in a laboratory setting. These viruses are modified to be cold-adapted, meaning they are temperature-sensitive and can only replicate efficiently in the cooler environment of the nasal passages. The virus strains are restricted from growing in the warmer temperatures of the human lungs and lower respiratory tract, which helps prevent them from causing full-blown illness.
This method of delivery induces a comprehensive immune response that includes both systemic and mucosal protection. The replication of the weakened virus in the nose stimulates the production of secretory immunoglobulin A (IgA) antibodies directly at the site of viral entry, known as mucosal immunity. This localized protection creates a strong first line of defense against infection, which is an advantage compared to the injectable inactivated vaccine. The intranasal application also stimulates a cellular immune response, potentially offering broader protection against different flu strains.
How Vaccine Effectiveness is Measured
The protective power of any influenza vaccine is assessed using two distinct metrics: vaccine efficacy and vaccine effectiveness (VE). Vaccine efficacy is a measure determined in highly controlled clinical trials, typically randomized controlled trials, where the vaccine is tested under ideal conditions against a placebo. These trials measure the percentage reduction in the risk of illness among vaccinated individuals compared to those who received a placebo.
Vaccine Effectiveness (VE), in contrast, measures how well the vaccine works in the real world, where factors like patient health status, vaccine storage, and the match between the vaccine strains and circulating viruses are not controlled. The primary method used for estimating VE each season is the test-negative design. This observational study involves testing patients who seek medical care for acute respiratory illness for the presence of influenza. VE is calculated by comparing the odds of vaccination in patients who test positive for the flu (cases) versus those who test negative (controls).
The History of FluMist Efficacy Concerns
The effectiveness of FluMist was a subject of concern, especially during the mid-2010s, after a period of unexpectedly low performance. For several influenza seasons, particularly between 2013 and 2016, data from the United States indicated that the nasal spray vaccine provided significantly lower protection compared to the traditional flu shot in children. The Advisory Committee on Immunization Practices (ACIP) initially recommended FluMist as the preferred option for young children before the 2009 H1N1 pandemic, but this changed as the data shifted.
The failure was most pronounced against the influenza A/H1N1 strain, which was circulating widely during the 2013–2014 season. While the inactivated vaccine showed moderate effectiveness against H1N1, FluMist demonstrated a near-zero protective effect against this specific strain in children. This led to the ACIP recommending against the use of FluMist in the United States for the 2016–2017 and 2017–2018 seasons.
Scientific investigation focused on why the H1N1 component of the LAIV was failing to generate a sufficient immune response. One hypothesis was that the H1N1 vaccine virus strain had a reduced ability, or “fitness,” to replicate in the nasal passages of recipients, hindering immune stimulation. Reformulation efforts were undertaken, including changing the specific H1N1 component strain; however, subsequent studies showed that even the updated strain did not restore the vaccine’s effectiveness to acceptable levels in children.
Current Use and Recommendations
Following the periods of concern, FluMist was reintroduced with new data supporting its use, and it is now considered an acceptable option for certain populations. The vaccine is currently approved and recommended for healthy, non-pregnant individuals between the ages of 2 and 49 years. It offers an alternative for those who prefer to avoid an injection.
However, the live nature of the vaccine means it is not suitable for everyone, and several contraindications exist. The nasal spray should not be administered to children aged 2 through 4 years who have asthma or a history of wheezing in the past year, due to an increased risk of wheezing after vaccination. It is also not recommended for pregnant women, individuals with severely weakened immune systems, or those with certain chronic medical conditions, such as lung, heart, or metabolic disorders. For most individuals, the inactivated influenza shot remains the most broadly applicable and preferred vaccine.