A secondary infection is one that occurs when a different infection, known as a primary infection, has made a person more susceptible to disease. It is called a secondary infection because it occurs either after or because of another infection.
There are several ways that a primary infection can increase susceptibility to disease, leading to a secondary infection.
Some diseases can lower the immune system's ability to fight off harmful invaders like viruses and bacteria. This can make it easier for them to enter and cause a secondary infection.
When a disease modifies the body's immune response to the point that the person becomes immunocompromised (AIDS, cancer, etc.), such secondary infections are referred to as opportunistic infections. Opportunistic infections are pathogens that in healthy/immunocompetent individuals do not normally cause disease, but in those with weakened immune systems can be more common or lead to serious complications.
Skin infections can compromise the skin's ability to act as a barrier to the outside world, particularly when they also cause breaks or sores. These can act as entry points for new infections.
Many common treatments for primary infections (such as antibiotics), can leave individuals vulnerable to developing secondary infections. Antibiotics are helpful when treating bacterial infections because they target and kill bacteria. However, in doing so, they kill off both the bad and the good - including normal, healthy flora (bacteria) that live in our bodies. Antibiotics can resolve the primary infection, but in doing so, the loss of the natural bacteria can open up the opportunity secondary infections.
Secondary Infections vs. Co-Infections
Secondary infections occur after, or because of, primary infections. However, sometimes people have multiple infections that aren't directly related to one another at the same time. These infections are often considered to be co-infections rather than secondary infections.
Secondary infections post-COVID-19 have been reported since the beginning of the pandemic, however in recent months the frequency and type of secondary infections have increased. Some of these secondary infections are becoming increasingly life-threatening, especially in vulnerable populations.
Recent research has highlighted the long-term damage and dysfunction that SARS-CoV-2 has on the immune system, even after mild COVID infections. It has been previously documented that COVID-19 infection can cause long-lasting immunocompromised state, likely from T-cell activation and exhaustion.
Researchers reported in a study published in April 2023 in the journal Immunity, that their "findings suggest that SARS-CoV-2 infection damages the CD8+ T cell response, an effect akin to that observed in earlier studies showing long-term damage to the immune system after infection with viruses such as hepatitis C or HIV."
The risks of secondary infections from SARS-CoV-2 increases with subsequent COVID infections as immune dysfunction increases.
Current research has so far identified multiple bacterial and fungal infections that are directly attributed to COVID-19 infection, and many more that have circumstantial evidence that suggests that COVID-19 may be associated with the infections.
The COVID-19 pandemic has likely altered other pathogens, including bacteria and fungi, potentially increasing the incidence of antimicrobial resistance. During the COVID-19 pandemic, there were improper uses of antimicrobials either in healthcare institutions or in communities, which in turn played a role in the increase in antimicrobial resistance.
It has been documented that about 72% of COVID-19-admitted patients were treated with antimicrobials, whereas solely 8% of these patients had bacterial or fungal co-infection. Additionally, antimicrobials were used early in the pandemic in the absence of other therapeutics on many severe hospitalized COVID-19 patients with which there were desperate attempts to find symptomatic relief for these patients. On top of the inappropriate use of antimicrobials, the increased and universal use of biocides (disinfectants) probably encouraged more indirect increases in antimicrobial resistance.
Since early 2020, this situation has expanded globally and might have supported the evolution of extremely resistant microorganisms, which might have played a critical role in worsening the status of some patients, especially those who were admitted to intensive care units (ICUs). Many patients with severe COVID-19 infections developed deadly bacterial and fungal secondary/co-infections, often referred to as "superinfections".
A systematic review published in 2022 in the International Journal of Environmental Research and Public Health found that the most commonly reported resistant Gram-negative bacteria were:
A. baumannii and K. pneumonia were highly resistant to tested antibiotics compared with E. coli and P. aeruginosa. Moreover, K. pneumonia showed high resistance to colistin.
The most commonly reported Gram-positive bacteria were:
The resistance of E. faecium to ampicillin, erythromycin, and Ciprofloxacin was high.
The authors concluded that self-antibiotic medication, empirical antibiotic administration, and antibiotics prescribed by general practitioners were the causes of increased antibiotic resistance during the COVID-19 pandemic.
A study published in PLOS Global Publich Health in 2022 of 318 COVID patients found that antibiotics were prescribed were heavily prescribed for all COVID-19 patients, with severe disease correlated to increased usage:
One of the mechanisms for the resistance is fungi tend to produce elevated amount of enzymes that are the main target for azole group and other antifungal drugs, hence preventing the suppression of metabolic reactions. Another mechanism used is the modification of the arrangement of targeted enzyme that minimize the attachment efficacy of azole significantly.