We tend to say we “caught the flu” this season. But there’s more than one flu virus. Because mild and asymptomatic infections can occur with influenza viruses, and because the various viruses differ significantly, according to our immune system, is it possible that we could suffer more than one type of flu infection during the same flu season?

Seasonal circulating influenza viruses

The interventions many countries rolled out against influenza, most especially the limitation on travel and contact, removed from seasonal circulation one of the two influenza B virus lineages we used to be infected by – B(Yamagata).

Today, we have three main seasonal influenza viruses circulating each year: influenza subtypes A(H1N1) and A(H3N2), and the remaining B lineage, B(Victoria).

While influenza C and D viruses also exist, they are not seasonal viruses among humans.

These influenza A & B types, subtypes and lineages also have emerging genetic variations referred to as “clades” and “subclades”, and we can see these using the same powerful genome sequencing techniques used to show SARS-CoV-2 evolution-driven genetic changes.

For the sake of this blog, I’m just going to call any patient-detected viruses that fall into genetic subgroupings “variants” because I think that conveys what they are to a broader audience, given what we became familiar with during the peak of the pandemic’s daily updates.

So, having heard one of my children sniffing this morning, I was thinking about the likelihood that we do get infected more than once by different influenza viruses, in the same season. So I had a quick look at the literature.

There is evidence of multiple flu infections each season

The study below supports that this is possible. The best studies for this sort of work are ones that take samples from the same group (cohort) of people over a long time. They are called longitudinal studies.

The Ha Nam Household Cohort Study

A report published in 2012 described the findings from a longitudinal household cohort study in Vietnam, which examined randomly chosen households for the presence of influenza-like illness (ILI), defined as illness with a temperature of 38 °C or higher and a cough or sore throat. The study of an unvaccinated population ran across three consecutive seasons from December 2007 to April 2010. Among 427 participants assessed over all three seasons, 242 (57%) had at least one ILI, and between 17% to 26% had one flu virus-positive ILI each season.

This study also identified flu virus variant changes between seasons. A limitation of this study is that it will underestimate flu virus infections because it doesn’t include non-ILI events, of which there may be many.

For A(H1N1), the variant changes:

• Season 1: A/H1N1/Brisbane/59/2007-like
• Season 2: A/H1N1/ Brisbane/59/2007-like
• Season 3: A/H1N1/California/7/2009-like

For A(H3N2), the variant changes were..

• Season 1: A/H3N2/Brisbane/10/2007-like
• Season 2: A/H3N2/Perth/16/2009-like

The study found multiple infections within the same season among 27 individuals (11% of ILIs), which included 23 with two types/subtypes and four with all three types/subtypes. The investigation did not discriminate influenza B into lineages. Among the 27 people, intraseasonal infection patterns detected by either RT-PCR or evidence of seroconversion (antibody levels increasing) included:

• 6 – A(H1N1) and A(H3N2)
• 9 – A(H1N1) and influenza B
• 8 – A(H3N2) and influenza B
• 4 – A(H1N1) and A(H3N2) and B A(H1N1)

Interestingly, and numbers are small, it looks (no stats used here!😬) like influenza B may be more often involved in these intraseasonal multiple flu virus infections than two influenza As. A third involves two fluAs (37%), while two-thirds (78%) include a fluB. Whether that is meaningful can’t be shown here.

Takeaway point…

The key point here is that you can be infected and made ill by more than one influenza type and subtype in a given season. So just because you got sick once this winter, don’t think you’re safe! Oh, and influenza isn’t the only virus around – apart from SARS-CoV-2 (that causes COVID-19), there are nearly two hundred distinct rhinovirus types, as well as multiple adenoviruses, parainfluenzaviruses, respiratory syncytial viruses, human metapneumoviruses, enteroviruses, parechoviruses and bocaviruses.

To reduce your risk of severe illness, consider influenza vaccination (and COVID-19 and any other vaccines you’re not up-to-date with) to help moderate the severity of the disease if you do get infected. Also, wearing a tight-fitting N95 or P2 (or higher filtration quality) mask that covers your nose and mouth whenever you are sharing air with people protects you and them if you’re shedding.

A mild infection for you may not be for someone else.