The live attenuated influenza vaccine (LAIV) offers a distinct approach to flu prevention compared to traditional injections. This vaccine contains a weakened, live version of the influenza virus, which has been modified to prevent it from causing illness in healthy individuals. Unlike the flu shot, which is administered as an injection, LAIV is delivered directly into the nose as a spray. This method of administration and the vaccine’s unique composition are central to its function and how it prompts the body’s protective response against influenza.
How the Nasal Spray Flu Vaccine Works
The effectiveness of the nasal spray flu vaccine stems from “attenuation,” where scientists weaken the influenza virus so it cannot cause severe illness in healthy people. This weakened virus is “cold-adapted” and “temperature-sensitive,” meaning it multiplies only in the cooler temperatures of the nasal passages and upper respiratory tract, not in the warmer lungs or other parts of the body where influenza typically causes disease.
Once sprayed into the nose, these attenuated viruses replicate locally, mimicking a natural infection without triggering full flu symptoms. This replication stimulates a robust immune response, including the production of antibodies and immune cells. A notable aspect is the induction of mucosal immunity, which provides localized protection at the primary entry point for the flu virus. This immune activation helps the body recognize and fight off future encounters with the actual influenza virus.
Key Differences from the Flu Shot
The live attenuated influenza vaccine (LAIV) differs from the inactivated influenza vaccine (IIV), commonly known as the flu shot. The most apparent distinction lies in their administration method: LAIV is given as a nasal spray, a needle-free option, while the flu shot is delivered via an intramuscular injection, typically into the arm.
Their composition is another difference. LAIV contains a live but weakened form of the influenza virus, which replicates in the upper respiratory tract. In contrast, flu shots contain inactivated, or “killed,” influenza viruses or components that cannot replicate. These structural differences lead to varied immune responses. LAIV creates a broader immune response that includes mucosal immunity in the nasal passages, the initial site of viral entry and replication. While both vaccines elicit systemic antibody responses, LAIV’s ability to mimic a natural infection more closely can lead to a more diverse T-cell response and potentially superior functional antibodies.
Who Is Eligible for the Live Attenuated Vaccine
The live attenuated influenza vaccine (LAIV) has specific guidelines regarding who can receive it. Generally, it is approved for healthy, non-pregnant individuals between 2 and 49 years of age.
However, there are several contraindications that exclude certain individuals from receiving LAIV. It is not recommended for pregnant individuals or those with weakened immune systems due to disease or medication, as the live virus, even attenuated, could pose a risk.
Children aged 2 through 4 years with a history of asthma or wheezing in the past 12 months should not receive LAIV, nor should anyone 5 years or older with asthma. Individuals who have taken influenza antiviral medication in the last three weeks or have a history of severe allergic reaction to a flu vaccine should also avoid LAIV.
Effectiveness and Potential Side Effects
The effectiveness of the live attenuated influenza vaccine can vary each flu season, depending on how well the vaccine viruses match the circulating influenza strains. Studies have shown that LAIV provides significant protection against medically attended influenza illness. While its effectiveness can be comparable to inactivated vaccines against certain influenza A and B viruses, some past seasons, particularly those dominated by the 2009 pandemic H1N1 virus, saw LAIV being less effective than inactivated flu vaccines in children.
LAIV causes mild and temporary reactions. Common side effects in children include runny nose or nasal congestion, headache, sore throat, and cough. Adults may also experience similar symptoms, along with muscle aches and fever. These reactions are a sign that the immune system is building protection. More serious adverse events are rare, and studies have shown no significant difference in the incidence of serious complications like pneumonia or bronchitis between LAIV recipients and placebo groups.