Mice study didn’t show flu vaccine causes strep A

A post on Facebook claims that nasal flu vaccines given to children “is where the strep A infections pandemic in the kids is coming from”. 

The post consists of a screenshot of a study published on the US site PubMed, titled “Live attenuated influenza vaccine enhances colonization of Streptococcus pneumoniae and Staphylococcus aureus in mice”. 

A caption written by the author states: “Stop giving your kids the flu shot. In fact stop giving them anything right now. The government cannot be trusted.”

Very similar claims linking the flu vaccine and strep A have been shared many times online since the current strep A outbreak gained widespread attention in December, and we have fact checked them a number of times before. 

These claims are false. The study investigated a different type of bacteria—not strep A— and found that nasal flu vaccines did not increase the risk of severe infections.

Strep A and the flu vaccine

Group A streptococcus (or strep A) is a common bacteria that can be responsible for a range of diseases, ranging from mild to severe. Scarlet fever, a disease most common in children, is caused by the bacteria and can lead to symptoms including fever, sore throat, rash and swelling of the tongue. 

If the bacteria enters deeper into the body, such as the lungs or bloodstream, a strep A infection can develop into an invasive group A strep (iGAS) infection. These can be potentially life-threatening. 

The UK Health Security Agency (UKHSA) has reported a higher rate of scarlet fever and iGAS infections in recent months. As of 8 January 2023, there have been 190 iGAS deaths across all age groups in England this season, including 30 children.

Pre-print research published by the UKHSA in December found a reduction in strep A infections in areas where more children received the nasal flu vaccine, though it said “there was no difference in scarlet fever or invasive group A strep (iGAS) notifications.”.

A UKHSA spokesperson told Full Fact last month: “The flu vaccine reduces the risk of having flu and group A Strep infections at the same time therefore it reduces the chance of developing invasive Group A Strep.

“We strongly recommend children who are eligible get a flu vaccine – it’s the best way to protect them from serious illness. The flu vaccine for children has an excellent safety record, this includes the nasal spray given to school aged children and pre-schoolers, which has been given to millions of children in the UK and worldwide.”

What does the study say?

The study in question, originally published by the scientific journal mBio, found that the live attenuated flu vaccine (the same type given to children in the UK) increases the likelihood of finding two types of bacteria—including two strains of streptococcus pneumoniae—in the upper respiratory tract of mice. 

This doesn’t mean that the nasal flu vaccine leaves children more susceptible to strep A infections. They may sound similar, but streptococcus pneumoniae and strep A (group A streptococcus) are different types of bacteria. 

Streptococcus pneumoniae is the name given to a group of more than 90 different strains of bacteria which can cause non-invasive infections such as bronchitis, as well as invasive infections such as pneumonia and meningitis. The study looked at two of these strains, and did not find any evidence of an increased risk of severe infection or death caused by streptococcus pneumoniae in mice given the vaccine.

The study also highlights that while flu vaccination with live attenuated vaccines “primes the upper respiratory tract for increased bacterial growth and persistence of bacterial carriage” in a very similar way to an actual flu virus infection [sometimes referred to as wild-type influenza], it doesn’t increase severe bacterial disease in the lower respiratory tract in the way an actual flu virus infection can.