IJID: Short-Term Risks of Influenza and COVID-19 Following Influenza Infection: A Self-Controlled Case Series Study

When Epidemic Viruses Collide

#18,953

Viral interference is the ability of one circulating virus to inhibit or impede the transmission of another - and while frequently observed, it remains only partially understood.

Sixteen years ago, during the 2009 H1N1 pandemic, some countries that reported rampant rhinovirus outbreaks in the fall saw far less H1N1 activity than expected (see 2009 New Scientist article Common cold may hold off swine flu).

In 2020's When Epidemic Viruses Collide, we looked at a report in The Lancet Microbes which suggested active Rhinovirus infection may temporarily block infection with the influenza A virus (see Interference between rhinovirus and influenza A virus: a clinical data analysis and experimental infection study)

A year later we saw a similar interaction reported by researchers from the University of Glasgow (see Study: Human Rhinovirus Infection Blocks SARS-CoV-2 Replication Within the Respiratory Epithelium) who found at least one common Rhinovirus (A16) inhibited infection of (in vitro) human respiratory epithelium cells by the SARS-CoV-2 virus.

While the exact mechanism behind this subduing of competing viruses is an ongoing topic of research, many researchers believe that exposure to one virus activates not only a targeted immune response - but also the body's innate immune response - essentially raising `shields' against other possible viral invaders.

A heightened immune response that may persist for weeks or even months; an idea that has been dubbed the `temporary immunity hypothesis'.

In 2017's PLoS Comp. Bio.: Spring & Early Summer Most Likely Time For A Pandemic, researchers used `viral interference' and/or temporary immunity to help explain why pandemics typically emerge in the spring or early summer; after the end of regular flu season.

During the first two years of COVID, Influenza A all but disappeared; but how much of that was due to viral interference - and how much was due to social distancing and face mask wearing - is difficult to parse out. 

But over the past 3 years, influenza A has made a strong comeback - and while we've seen concerns raised each winter over seeing a `twindemic' - so far, that has not materialized.  

Providing some answers as to why that might be, we have a study from Beijing that found that influenza A infection reduces the risk of influenza reinfection by 57% for up to 8 months but increases the risk of COVID-19 infection by 48% within 6 months post-influenza.

While this points to an increased risk of COVID infection following a bout of influenza (Flu-COVID), they did not study the reverse (and admittedly far more complex) COVID -> Flu scenario. Other notable limitations include:

• Data was geographically and temporally limited (Beijing, China, mostly from 2023)
• they did not take into account prior vaccination or infection history
• and they did not include non-medically attended asymptomatic or mild cases 

Still, the data on increased COVID susceptibility following influenza season is compelling, which may help inform public health planning going forward. 

And those who experience a bout of influenza just might want to consider taking more stringent preventive measures (masking, social distancing, COVID booster) in the months that follow.

I've just posted the abstract and a few excerpts, so follow the link to read the study in its entirety. 

Short-Term Risks of Influenza and COVID-19 Following Influenza Infection: A Self-Controlled Case Series Study

Chao Wang1,2,3 ∙ Yanlin Gao1,2 ∙ Wei Li1,2 ∙ … ∙ Yunping Shi1 ∙ Ying Zhou1 ∙ Gang Li1,2 ligangcn@126.com … Show more

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Highlights

• Surveillance data from Beijing revealed that COVID-19 peaks occurred 2-4 months after influenza epidemics. 

• Influenza infection reduces the risk of reinfection by 57%, with protection lasting up to 8 months.

• Influenza infection increases the risk of COVID-19 by 48% within 6 months.

• COVID-19 testing and preventive measures during and after influenza seasons should be enhanced.

ABSTRACT

Objectives

Given the inconsistent existing evidence on viral interference between influenza and COVID-19, this study aimed to investigate the short-term risks of influenza reinfection and COVID-19 infections following an initial influenza episode, in the context of cocirculating influenza and SARS-CoV-2 viruses.

Methods

Using a seasonally adjusted self-controlled case series (SCCS) design, we analysed surveillance data from Beijing, China, collected between February 2023 and June 2025. The study included individuals infected with influenza who subsequently experienced either influenza reinfection or COVID-19 within 18 months after the initial influenza infection.

Results

The study comprised 17,859 individuals with influenza followed by COVID-19 (flu-COVID) and 34,291 with sequential influenza infections (flu-flu).

Influenza infection was associated with a transient increase in the risk of COVID-19 within 6 months (IRR = 1.48, 95% CI: 1.39–1.59).

In contrast, the risk of influenza reinfection was significantly reduced during the same period (IRR = 0.43, 95% CI: 0.39–0.49). Although risk modulation varied across demographic subgroups, results from subgroup and sensitivity analyses were largely consistent.

Conclusions

Influenza infection may temporarily increase susceptibility to COVID-19 while reducing the risk of influenza reinfection. These findings highlight the importance of targeted surveillance and preventive strategies during influenza peaks to mitigate subsequent COVID-19 burden.

       (SNIP)

Discussion

Our study revealed markedly divergent short-term risks of SARS-CoV-2 and influenza reinfection within six months following an initial influenza infection. Specifically, compared to the period 7–18 months post-infection, the risk of COVID-19 increased by 48%, while the risk of influenza reinfection decreased by 57%. This pattern suggests that adaptive immune memory—developed through prior natural infection or vaccination—can effectively prevent reinfection or attenuate symptom severity and complications[22].

Previous research has indicated that immunity conferred by influenza vaccination typically persists for approximately six months[20–21]. Our results extend this understanding by showing that natural infection may confer protection against influenza reinfection for up to eight months.

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