Factors Influencing Vector Shedding

Vector shedding is the release of virus-based gene therapy products from the patient through one or all of the following routes: excreta (faeces), secreta (urine, saliva, nasopharyngeal fluids, etc.), and skin (pustule, sores, wounds)1


Biological Characteristics of Viral Vector

A number of different biological characteristics of the viral vector can impact vector shedding1

Replication Competence1

The ability of the viral vector to replicate can greatly affect how it is disseminated in the body

This may increase the extent and duration of shedding

Vectors that are replication competent may have a longer shedding period than replication-incompetent or deficient viral vectors


Gene therapies that are derived from viruses that elicit a strong immune response may be cleared from circulation more rapidly than less immunogenic therapies

These vectors may also be shed for a shorter duration

Persistence and Latency*1,2

Viral vectors that exhibit persistence or latency reactivation in the host may have unpredictable and intermittent shedding

For example, an oncolytic herpes virus product that is capable of latency has a longer duration of shedding

This is not relevant for replication-deficient viruses, since they cannot reactivate

*Period of time during which a virus is present in the host without producing overt clinical symptoms.1


Tropism of the gene therapy product can influence the route through which the vector is shed

Gene therapy products that are engineered to carry tropism-modifying genes or mutations may exhibit a shedding profile that is different from the parent virus because of re-targeting of the product to a different tissue or organ

The altered tropism can influence the type of clinical samples used to study vector shedding

Stability of Product Attentuation1,3,4

It is common for viral-based gene therapy products to be attenuated* either for replication in normal cells or for loss of virulence or latency in the host

The stability of these attenuating modifications can alter the pathogenicity of viral-based gene products

Though this has not been documented, there is the possibility that an attenuated viral-based gene product may lose the attenuating modification by recombination of the viral vector with the wild-type parental virus in the target cell

This can alter the shedding pattern and/or components that are shed

For replication-deficient viral vectors that are devoid of most of the viral sequences, the probability of this happening is low

*Attenuating modifications make a vector replication defective.3

Route of Vector Administration

The route of vector administration influences excretion routes through which vectors may be shed1,5

Administration of viral-based gene therapy products through local routes (e.g. intraperitoneal or ocular) is less likely to result in vector shedding in bodily fluids or excreta compared with systemic administration5

However, even with local administration of viral-based gene therapy products, shedding can occur at the site of injection1,6

*Please refer to local prescribing information.

AAV, adeno-associated virus; RPE65, retinal pigment epithelium-specific 65 kDa protein gene.


Subretinal injections of an AAV vector in patients with confirmed biallelic RPE65 mutation-associated retinal dystrophy resulted in vector shedding in tears and serum*6

Dose of Administered Vector

Dose of administered vector can impact magnitude and duration of shed vector5,7

Higher doses of the viral-based gene therapy product can result in more vector being shed7,8

AAV, adeno-associated virus.


Patients with severe haemophilia B who received a single infusion of a high dose of an AAV8 vector had the highest levels of vector genome, with mean plasma levels that were 20,000-fold higher than those in patients receiving the low dose7,8

Time Since Vector Administration

Shedding is most likely to occur in a period immediately following product administration1

However, the shed vector can potentially be detected for several weeks post treatment8

With replication-competent products, a second peak of shedding may be observed a few days/weeks after administration due to its amplification in vivo1

However, since most viral-based gene therapy products are replication incompetent, this second peak of shedding should not occur1

AAV, adeno-associated virus.


In patients with severe haemophilia B, a single intravenous infusion of an AAV8 vector resulted in the presence of vector genome in saliva, urine, semen, stool, or plasma for up to 6 weeks post treatment8


      1. U.S. Food and Drug Administration. Design and analysis of shedding studies for virus or bacteria-based gene therapy and oncolytic products. Available at: https://www.fda.gov/regulatory-information/search-fda-guidance-documents/design-and-analysis-shedding-studies-virus-or-bacteria-based-gene-therapy-and-oncolytic-products. Accessed 13 May 2022.
      2. Burton EA, et al. Stem Cells 2001;19(5):358–377.
      3. European Medicines Agency. Guideline on scientific requirements for the environmental risk assessment of gene therapy medicinal products. Available at: https://www.ema.europa.eu/en/documents/scientific-guideline/guideline-scientific-requirements-environmental-risk-assessment-gene-therapy-medicinal-products_en.pdf. Accessed 13 May 2022.
      4. Baldo A, et al. Curr Gene Ther 2013;13(6):385–394.
      5. Salmon F, et al. Expert Rev Clin Pharmacol 2014;7(1):53–65.
      6. Voretigene neparvovec® [Product Information]. 2021. Available at: https://www.ema.europa.eu/en/documents/product-information/luxturna-epar-product-information_en.pdf. Accessed 13 May 2022.
      7. Nathwani AC, et al. N Engl J Med 2014;371(21):1994–2004 (supplementary appendix).
      8. Nathwani AC, et al.N Engl J Med 2014;371(21):1994–2004.