Fact check: Nasal flu vaccine does not push through the cribriform plate to the brain

An image shared online has made several false or misleading claims about the safety of the nasal flu vaccine, which is routinely offered to children in Britain.

Reuters Fact Check. REUTERS

The image mimics an informational poster, and can be seen here (here).

The nasal spray flu vaccine, also known as the flu mist, contains live flu viruses that have been weakened to stimulate the immune system while not causing disease. It is given as a spray that is squirted up the nostril (here).

Text on the image claims that the flu mist vaccine “pushes through” the cribriform plate, which forms the roof of the nasal cavity (here), to the brain. This is false.

AstraZeneca, the developers of FluMist® Quadrivalent (www.flumistquadrivalent.com/), told Reuters that since the vaccine is administered via the nasal passages it is “essentially a topical application”. This means it does not “push through the cribriform plate” as stated, nor does it reach the brain.

“It triggers your immune system to build defenses (called antibodies) in the nose where we are typically exposed to flu and throughout the body to help fight against the flu virus”, a spokesperson for the company said.

Reuters has previously reported that the brain cannot be accessed from the nasal cavity (here) .

At the time, a spokesperson for Public Health England (PHE) said that: “There is no point in the respiratory tract or nasal cavity where the brain is accessible. It would not be possible to touch the brain with a swab without drilling through the cribriform plate.”

The text on the misleading flu vaccine poster goes on to warn about viral shedding, which it says happens for 28 days after inoculation, with “peak shedding” on day three.

Public Health England (PHE) explains that although vaccinated children can shed the vaccine virus a few days after vaccination, this weakened virus is less able to spread than the natural infection (here) .

The amount of virus shed is also normally too low to infect others, and the virus does not survive for long outside the body.

According to the U.S. Centers for Disease Control and Prevention (CDC), studies have shown that the maximum shedding of Live Attenuated Influenza Vaccine (LAIV) occurs within 2 days of vaccination, but in most cases stops 11 days after vaccination (tinyurl.com/y56p469h) .

The poster lists what it alleges are adverse effects of the vaccine that are written on the manufacturers packaging. These include meningitis, anaphylactic shock and vomiting.

The brand name of the nasal flu spray used in the UK is Fluenz Tetra. The potential side effects are listed (here) and range from a runny or stuffy nose to uncommon reactions like nose bleeds.

The claim that the vaccine “has not been proven to be safe or effective” is not supported by health bodies like UK’s National Health Service, which advises that: “The nasal spray flu vaccine for children is very safe” and that “the side effects linked with the nasal spray flu vaccine are almost always mild and short-lived” (here).

This is echoed by the Vaccine Knowledge Project, a website managed by the Oxford Vaccine Group, an academic research group within the University of Oxford, which points to the “excellent safety record” of flu vaccines (here).

Research from Public Health England suggests that the vaccine has reduced flu in the population. Citing this research, the Vaccine Knowledge Project, says that across all age groups including children, the flu vaccine prevented 15-52% of flu cases between 2015 and 2020 (tinyurl.com/yy97ks3c ).


False. The nasal flu vaccine does not push through the cribriform plate to the brain. While shedding of the vaccine virus does happen, unvaccinated people are not at risk of becoming seriously ill with the vaccine virus. The vaccine has a good safety record and side effects linked to it are almost always mild.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts here .

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