This idea came to my mind when thinking about how many people become ill in many locations with similar symptoms (fever and rough cough) from Influenza.

There are of course different types of influenza virus.

Can Influenza A virus work as a vector for other upper respiratory infections? That is, much the same as the mosquito being a vector of malaria, can an influenza virus carry other infections with it?

If so, how? If not, why do some people get so sick from viral influenza, and not others?

  • $\begingroup$ What do you mean by vector? That it can cause other respiratory infections? $\endgroup$ – Chris Dec 10 '14 at 16:21
  • $\begingroup$ @Chris No. That it can help transport other respiratory infections which cause other respiratory infections. $\endgroup$ – Léo Léopold Hertz 준영 Dec 10 '14 at 16:33
  • $\begingroup$ Transport in the way that it helps a secondary infection to get into the upper respiratory pathways because the immune system is already dealing with influenza? $\endgroup$ – Chris Dec 10 '14 at 16:36
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    $\begingroup$ I think the meaning is much the same as the mosquito being a vector of malaria, "can an influenza virus carry other infectors with it?" @chris $\endgroup$ – rg255 Dec 10 '14 at 16:39
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    $\begingroup$ I don't think there is a reason to close this question. I think it's better to ask for clarification first. $\endgroup$ – Chris Dec 10 '14 at 16:57

I wouldn't call influenza a vector here, but yes, an infection with the flu virus which results in a full infection can have secondary infections as complications. There are often caused by bacteria which usually live in the respiratory pathway and kept under control by the immune system.

This includes pneumonia (either viral or bacterial), inflammation of muscles and the heart muscle (myositis and myocarditis) as well as pericarditis. Less severe complications include infections like ear infections and sinusitis.

See the references for more details:


  1. Complications of Viral Influenza
  2. Double trouble: Bacterial super-infection after the flu
  3. A Mouse Model of Lethal Synergism Between Influenza Virus and Haemophilus influenzae
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  • $\begingroup$ Does this help answering the question? If not, you need to be more specific and I will work on it later today. $\endgroup$ – Chris Dec 10 '14 at 16:57
  • $\begingroup$ It helps. Is it possible that influenza is asymptomatic but it causes opportunistic diseases? This can be one explanation - influenza triggering some infections of upper respiratory tract without itself causing any symptoms. This could explain why influenza is felt differently by different people too. $\endgroup$ – Léo Léopold Hertz 준영 Dec 10 '14 at 18:06
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    $\begingroup$ I don't know right at the moment, but I will look into it. The other problem is the use of the word flu - most people which talk about having a "flu" in fact have something else. Most likely an influenzal infect (which is not caused the the influenza virus). This one feels bad as well but is not as severe and clears much faster (usually within a week, from what I know from my MD colleagues). $\endgroup$ – Chris Dec 10 '14 at 18:42

A vector transmits a disease or parasite from one animal or plant to another. Fruit bats are vectors for Ebola. They are a host to the virus, which can then be passed on to humans.

Influenza A, the virus responsible for human seasonal epidemics, is not a vector for other pathogens, although co-infection with two different strains of influenza can give rise to progeny viruses with mixtures of genes derived from the parental strains. If anything, that would make humans, other mammals and birds the vectors for Influenza A.[1]

Influenza A doesn't carry plasmids, and it does not insert it's genes into cell DNA as does a retrovirus.

The most frequent serious complications of influenza are pulmonary and fall into 4 categories or groups:

  • primary influenza pneumonia
  • secondary bacterial pneumonia (also called co-infection, but from different sources)
  • pneumonia due to unusual pathogens or in immunocompromised hosts
  • exacerbations of chronic pulmonary diseases

Typically, the first group is smaller than the second (except for Avian Influenza A). No additional infection is necessary in this group to cause pneumonia or death.[2]

In the second (and largest) group, typically viral influenza infection is followed by near resolution of symptoms, subsequently complicated 4 to 14 days later by a recurrence of fever, dyspnea, productive cough, and pulmonary consolidation. Common pathogens are S. pneumoniae, S. aureus (methicillin-sensitive [MSSA] or methicillin-resistant [MSSA]), S. pyogenes, H. influenzae, and some others.

The pathogenetic mechanisms of viral-bacterial co-infections result from virus induced alteration in pulmonary epithelial cells (e.g. reduced ciliary function, cell death/decreased epithelial barrier function, etc.), virus-mediated inhibition of innate immune cells (e.g., suppressed phagocytosis, impaired microbial killing, etc.), dysregulated inflammation (e.g. increased inflammation due to chemokines) and other mechanisms.[2][3]

Pneumonia due to unusual pathogens or in immunocompromised hosts, and exacerbations of chronic pulmonary diseases are less common than the second group but carry a high mortality.

The second reference is an exhaustive review which you might find quite helpful.

1 FluGenome: a web tool for genotyping influenza A virus
2 Complications of Viral Influenza
3 The co-pathogenesis of influenza viruses with bacteria in the lung

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  • $\begingroup$ Yes, I like the second source most: Better use of influenza vaccines and antivirals may decrease risks of pulmonary complications, but novel prevention and treatment strategies are needed to improve outcomes for those at highest risk. This proposes me that all coinfections caused by influenza A virus are not known or cannot be protected by the usual influenza vaccine. Meta-analysis of the development of influenza vaccine would be great. I have the general idea that newer vaccines include just recent developed strains. I am not sure if this is best approach, since some do not have old ones. $\endgroup$ – Léo Léopold Hertz 준영 Dec 11 '14 at 12:47

While a viral infection such as influenza can lead to secondary infections, it is inappropriate to call it a vector for those other infections. It is not the means by which those other infections are acquired. If a bacterial sinus infection follows upon a bout with the flu, that bacteria was not carried to the patient by the influenza virus. A single bacterium is immense compared to any virus. No, the bacteria most likely resided within the host before the infection, but it was under control. Or it might be introduced during the infection and find a foothold it would not have likely found in a healthy host. In a situation like this, the immune system is distracted and weakened enough to present an opportunity for this bacteria to grow out of control into an infection.

The bacteria in this scenario are a little bit like the annoying hordes of roofing contractors that recently swarmed my neighborhood after a hail storm. These guys are always around, looking for the occasional roof that might be getting old and need repair. But right after a hail storm, they find a lot more of the opportunities they are looking for. So suddenly they are all over the place. They didn't literally fall from the sky with the hail. But the hail presented the opportunity they needed to thrive.

Malaria is a microscopic germ which hitches a ride inside a comparably enormous mosquito to find it's way eventually into a human blood stream. An influenza virus is too small for anything infectious to hitch a ride inside it.

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    $\begingroup$ love the roofers analogy $\endgroup$ – rg255 Dec 10 '14 at 17:45
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    $\begingroup$ Except the roofers are symbiotic. $\endgroup$ – vajra78 Dec 10 '14 at 21:56
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    $\begingroup$ @vajra78 OK. Most of these roofers are symbiotic. But let's say some are parasitic. They are extremely crooked, unlicensed fly-by-night businesses that take advantage of the home owner. They use substandard materials, and then disappear after collecting initial payment without even finishing the job. Symptoms may include drained funds, uncovered roofs, and water-damaged homes. They cannot thrive in an ordinary environment as reports of their misdeeds and lawsuits catch up with them. But after a big hail storm, the volume of need allows them a temporary foothold. $\endgroup$ – Mark Bailey Dec 10 '14 at 22:37

Contrarily to the main opinion expressed so far, I would say that, in theory, it is possible that an Influenza virus can be a vector for another virus.

Here is a paper about using influenza virus as vector for a variety of genes. The same method can be leveraged to engineer an Influenza virus particle that contains, for example, genes coming from another virus. Upon infection, the Influenza virus will infect the host cells with both genomes.

In this way you will end up with an Influenza virus that act as vector for another virus.

However, such a hybrid looks very unlikely to occur in nature.

But it can be certainly made in lab.

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  • $\begingroup$ Excellent article! Thank you for your answer! Please, add further articles and/or books if there are some which could be useful to understand this case better. $\endgroup$ – Léo Léopold Hertz 준영 Jun 2 '15 at 15:46
  • $\begingroup$ There are tons of similar articles for many different viruses. In molecular biology they are called viral vectors. Its a virus used to transfer DNA to a target cell. Just google viral vectors and you will have a nice overview. $\endgroup$ – alec_djinn Jun 2 '15 at 15:47

In bacteriophage, the viral capsules serve as vectors for other DNA so frequently that there is a common name for this: transduction. The image below, from the Wikipedia, illustrates the basic process. Here a phage, with pink DNA, accidentally packages up other DNA -- in this case, from the bacterium -- in one of the capsules and thereby serves as a vector for this DNA.

enter image description here

Something a bit like this happens any time we have recombinational assortment in the influenza virus. The influenza virus genome segmented, with 8 "chromosomes". When an individual is multiply infected with different strains, new combinations of the 8 chromosomes can be packaged up within an given viral particle. This process is often responsible for new epizootic strains.

In principle, genetic material from another virus could be packaged up accidentally into an influenza virus particle. If it happened to enter the right kind of cell, the influenza capsule could in that way act as a vector for the other virus's DNA. In practice, I would imagine that this would require remarkable coincidence and thus would be extremely uncommon in nature.

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  • $\begingroup$ +1 for picture and new proposal! Is this method of bacteriohpage the one of most likely methods how influenza could act as a carrier? Can you relate it to alec_djinn's answer? $\endgroup$ – Léo Léopold Hertz 준영 Jun 16 '15 at 10:01

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