Can You Get COVID and the Flu at the Same Time?

You can absolutely get COVID-19 and influenza at the same time, and the combination tends to be worse than either infection alone. A systematic review of early pandemic data found that about 0.8% of patients with confirmed COVID-19 also tested positive for influenza, though rates varied widely by region and season.¹ The phenomenon is real, documented, and clinically meaningful, but it plays out in ways that are more nuanced than a simple “double dose of illness.”

How Common Is Getting Both at Once?

Co-infection rates have never been enormous, but they are not negligible either. The 0.8% overall figure from a meta-analysis of early pandemic studies masks significant geographic differences: rates ran closer to 4.5% in Asia compared with roughly 0.4% in the Americas.¹ A later pooled analysis focused specifically on influenza A, the more common flu subtype, pegged the co-infection prevalence at about 2.5%.² The range across individual studies stretched from zero to over 50%, depending on the population sampled, how aggressively testing was done, and whether the local flu season was active.

That variation makes sense when you think about it. During the early years of the pandemic, masking, lockdowns, and social distancing suppressed flu circulation dramatically. Co-infection rates tracked closely with flu hospitalization rates in the United States, which means the number of dual infections climbed when both viruses were circulating heavily, such as during the 2021-2022 flu season.³ In other words, the risk of co-infection isn’t fixed. It depends on what is circulating in your community at any given time. When both viruses are peaking simultaneously, the odds go up.

Why Co-infection Tends to Be More Dangerous

Having both viruses at once is not simply a matter of stacking two sets of symptoms. The combination appears to intensify the disease beyond what either virus would cause on its own. Patients with co-infection have shown higher rates of respiratory distress, multi-organ dysfunction, and admission to intensive care compared with people who had COVID-19 alone.² Co-infected patients also faced a higher risk of needing mechanical ventilation and were more likely to experience acute kidney and cardiac injuries than those with COVID-19 alone.

Animal studies have helped explain why. In hamsters simultaneously infected with both viruses, the resulting pneumonia was more severe and longer-lasting than with either virus by itself.⁴ Similarly, mouse models showed that co-infection prolonged the period of active viral infection, increased the flood of immune cells into the lungs, and ramped up inflammatory signaling, all of which contributed to worse pneumonia and lung damage.⁵ Co-infection also impaired the body’s ability to produce neutralizing antibodies and certain immune cells that fight off the viruses, potentially leaving the person sicker for longer.

For context, even on its own, COVID-19 carries higher mortality risk than seasonal flu. A large comparative study found that COVID-19 was associated with roughly 1.8 times the odds of dying within 30 days compared with influenza, and that gap widened in younger adults and hospitalized patients.⁶ Adding a flu infection on top of that baseline risk is a particularly unwelcome combination.

How the Two Viruses Share Your Lungs

One of the more striking findings from animal research is that the two viruses do not actually infect the same cells at the same time. In co-infected hamsters, researchers found that lung tissue was divided into distinct zones: some areas showed the pattern of SARS-CoV-2 infection, while other areas showed the pattern of influenza infection, with the viruses never co-localizing in the same region.⁴ Even in the same bronchial tube, each virus occupied its own stretch of tissue. The nasal lining showed the same segregated pattern. Think of it less like two armies attacking the same fortress and more like two separate wildfires burning through different sections of the same forest. The total damage is worse because more of the lung is under assault, even though each virus sticks to its own territory.

The immune response to each virus also differs. Influenza tends to trigger a stronger early burst of certain antiviral signaling pathways, particularly interferon responses, than SARS-CoV-2 does.⁷ That difference may explain why the timing and sequence of infection matters as much as the co-infection itself.

Does the Order of Infection Matter?

It seems to, at least in animal models. Mice that caught influenza first and then SARS-CoV-2 a few days later experienced more rapid weight loss, worse lung damage, and a prolonged inflammatory response compared with mice that had only one virus.⁸ The sequential infection also worsened the brain-related complications that SARS-CoV-2 can cause. Interestingly, the flu-first mice actually produced less SARS-CoV-2 viral material, suggesting that the innate immune response kicked off by the flu infection partially suppressed SARS-CoV-2 replication. But that suppression was not enough to offset the extra inflammation and tissue damage.

Here is where things get interesting. When researchers replaced the live flu infection with a live-attenuated flu vaccine (the nasal spray type), they saw the same reduction in SARS-CoV-2 replication but without the increase in disease severity.⁸ The vaccine primed the immune system enough to put up a fight against SARS-CoV-2 without generating the collateral lung damage that a real flu infection causes. This is a mouse study, so the implications for humans are not direct, but the principle is compelling: the immune boost from vaccination can be protective in ways that actual infection is not.

Who Faces the Greatest Risk

Children and older adults show higher co-infection rates than the general adult population, and children with both viruses are more likely to need mechanical ventilation compared to those with COVID-19 alone.² That finding may surprise parents who think of COVID-19 as primarily an adult concern. Co-infections with SARS-CoV-2 and other respiratory viruses in general are more common in children than in adults, likely because kids are exposed to more novel pathogens and have less built-up immunity. The elderly, meanwhile, face the familiar double burden of weaker immune defenses and a higher rate of underlying conditions like heart disease, diabetes, and chronic lung problems that make any respiratory infection more dangerous.

People who are immunocompromised, whether from medication, chemotherapy, or conditions affecting the immune system, are also at elevated risk. Their bodies may struggle to mount an effective response to one virus, let alone two simultaneously. If you fall into any of these categories, the case for staying up to date on both flu and COVID-19 vaccines is especially strong.

How Doctors Test for Both

In the early months of the pandemic, testing for COVID-19 and flu typically required separate tests, which created practical problems in emergency departments and clinics that were already overwhelmed. That gap has been closed by multiplex testing platforms that can screen for SARS-CoV-2, influenza A, and influenza B from a single nasal swab in a single run. These panels have proven highly accurate, with clinical evaluations showing sensitivity above 97% for both SARS-CoV-2 and influenza A.⁹

A review of over 2,000 patient results using one such panel found that when a patient tested positive for a non-COVID respiratory pathogen, the odds of also having COVID-19 were low, with a negative predictive value for COVID-19 of 95%.¹⁰ That is a useful finding for clinicians making rapid decisions: a positive flu test makes a simultaneous COVID-19 infection unlikely, though not impossible. Newer assays have pushed sensitivity and specificity even further, with some capable of detecting as few as 10 copies of synthetic viral RNA while maintaining 100% specificity across multiple targets.¹¹

The practical takeaway: if you are sick enough to see a doctor during flu season, ask whether they can test for both viruses. The answer to which virus you have changes the treatment you should receive, and knowing whether you have one or both determines how aggressively your doctor should monitor you.

Treating a Dual Infection

The antivirals for COVID-19 and influenza are different drugs that work through different mechanisms, and having both infections means you may need both treatments. Case reports from China described older patients who were co-infected with SARS-CoV-2 and influenza A and treated with nirmatrelvir-ritonavir (the combination sold as Paxlovid) for COVID-19 alongside baloxavir marboxil (marketed as Xofluza) for influenza.¹² Both drugs were used simultaneously. Nirmatrelvir-ritonavir is a protease inhibitor that targets SARS-CoV-2 replication, while baloxavir blocks a different enzyme essential for influenza replication. Because they act on different pathways, combining them is pharmacologically straightforward, though clinicians still need to watch for drug interactions, particularly since ritonavir affects how the liver processes many other medications.

Timing matters with both drugs. Paxlovid works best when started within five days of COVID-19 symptom onset, and Xofluza or oseltamivir (Tamiflu) is most effective when started within 48 hours of flu symptoms. In a co-infection scenario, the window for effective treatment is narrow, which is another reason why early testing with a multiplex panel can make a real difference in outcomes.

Getting Both Vaccines at the Same Visit

One common question, especially during fall vaccination season, is whether it is safe or effective to get a COVID-19 booster and a flu shot on the same day. The evidence on this is encouraging. A study comparing people who received both vaccines concurrently versus on separate days found that getting both at the same time actually produced stronger and more durable antibody responses against SARS-CoV-2. Antibody levels against several SARS-CoV-2 spike variants were multiple-fold higher at both peak immunogenicity and six months later in the concurrent group compared with the separate group.¹³ The boost appeared to be in recall responses rather than brand-new immune reactions, meaning the vaccines were amplifying pre-existing immunity rather than generating a novel response.

A smaller Italian study examined co-administration of a third dose of an mRNA COVID-19 vaccine with a flu vaccine and found that the group receiving both simultaneously showed a different antibody trajectory than the group receiving only the COVID-19 vaccine.¹⁴ The picture is still being refined, but the overall trend from available data suggests co-administration is not only safe but may carry an immunological benefit, at least for the SARS-CoV-2 component. For flu protection, no significant reduction in effectiveness has been seen from getting both shots at the same time.

From a practical standpoint, the biggest barrier to vaccination is often scheduling. If getting both shots on the same day means you will actually get both instead of skipping one, that alone makes co-administration worthwhile. You might have a slightly more sore arm or feel a bit more fatigued for a day, but those side effects are minor compared with the risk of being unvaccinated against either virus during a season when both are circulating.

Bacterial Infections Add Another Layer

A complication that many people do not think about is the risk of bacterial co-infection on top of a viral respiratory illness. Historically, secondary bacterial pneumonia has been a major killer during flu pandemics. With COVID-19, the picture is somewhat different. One study comparing bacterial co-infection rates found that about a third of influenza pneumonia patients had an identified bacterial pathogen, compared with only about 10% of COVID-19 pneumonia patients.¹⁵ Bacterial co-infection is far less common in COVID-19 than in flu, which has implications for how aggressively doctors should prescribe antibiotics in each scenario.

That distinction matters because the early pandemic saw widespread prophylactic antibiotic use in COVID-19 patients, driven partly by the assumption that bacterial superinfection would be as common as it is in flu. The data suggests otherwise. For patients who have both COVID-19 and flu simultaneously, however, the bacterial superinfection risk may fall somewhere between the two, though direct data on this specific three-way scenario remains limited. If you are hospitalized with confirmed co-infection, your medical team will likely monitor closely for signs of secondary bacterial pneumonia.

How Recovery Differs Between the Two Viruses

Even after the acute illness resolves, COVID-19 and influenza leave different fingerprints on the immune system. A study that examined immune cells in the blood three months after recovery found that both post-COVID and post-flu patients had altered immune profiles compared to healthy controls, but the alterations were distinct. Post-COVID patients showed markedly increased expression of certain chemokine receptors on various immune cell types, suggesting ongoing immune activation or reorganization months after the virus cleared. Post-influenza patients showed different changes, primarily a decrease in a specific receptor on naive immune cells and monocytes.¹⁶

These findings reinforce what many people have experienced firsthand: COVID-19 tends to leave a longer and more complex immunological trail than the flu. Whether having both viruses at the same time amplifies or alters these lingering immune changes has not been specifically studied in humans yet, but given that co-infection already worsens acute outcomes, it would not be surprising if recovery were more complicated as well. The broader message is that respiratory viruses are not interchangeable. COVID-19 and flu look similar from the outside, with overlapping symptoms like fever, cough, and fatigue, but they interact with your immune system in fundamentally different ways, and those differences persist well beyond the week or two you spend feeling sick.

1
PubMed Central. COVID-19 and Influenza Co-infection: A Systematic Review and Meta-Analysis
2
PubMed Central. Pathogenesis of influenza and SARS-CoV-2 co-infection at the extremes of age: decipher the ominous tales of immune vulnerability
3
PubMed Central. SARS-CoV-2 and influenza coinfection throughout the COVID-19 pandemic: an assessment of coinfection rates, cohort characteristics, and clinical outcomes
4
Scientific Reports. Co-infection of SARS-CoV-2 and influenza virus causes more severe and prolonged pneumonia in hamsters
5
PubMed Central. Coinfection with SARS-CoV-2 and Influenza A Virus Increases Disease Severity and Impairs Neutralizing Antibody and CD4(+) T Cell Responses
6
International Journal of Infectious Diseases. Increased 30-day mortality risk in COVID-19 compared with seasonal influenza
7
Communications Biology. COVID-19 and influenza infections mediate distinct pulmonary cellular and transcriptomic changes
8
PubMed Central. Sequential Infection with Influenza A Virus Followed by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Leads to More Severe Disease and Encephalitis in a Mouse Model of COVID-19
9
PubMed Central. Clinical evaluation of a fully automated, laboratory-developed multiplex RT-PCR assay integrating dual-target SARS-CoV-2 and influenza A/B detection on a high-throughput platform
10
New Emirates Medical Journal. Flu Viral Multiplex Testing and its Implication for COVID-19 Testing
11
PubMed. Development of Multiplex Real-Time RT-qPCR Assays for the Detection of SARS-CoV-2, Influenza A/B, and MERS-CoV
12
PubMed Central. Antiviral Treatment in Older Chinese Patient with SARS-CoV-2 and Influenza A Virus Co-Infection: A Case Series
13
PubMed Central. Concurrent administration of COVID-19 and influenza vaccines enhances Spike-specific antibody responses
14
PubMed Central. Immunogenicity of third dose of anti-SARS-CoV-2 vaccine co-administered with influenza vaccine: An open question
15
PubMed Central. Bacterial Coinfection in COVID-19 and Influenza Pneumonia
16
PubMed Central. High-Dimensional Immunophenotyping of Post-COVID-19 and Post-Influenza Patients Reveals Persistent and Specific Immune Signatures After Acute Respiratory Infection