We Claim:
1. A method for preventing, treating, or ameliorating a complement-mediated disorder in a subject in need thereof, which comprises administering to the subject a recombinant viral vector comprising nucleic acid encoding Factor I, or a fragment or derivative thereof that retains C3b-inactivating and iC3b-degradation activity, such that a therapeutically effective amount of the encoded Factor ls or the fragment or derivative thereof, is expressed from the nucSeic acid in the subject, thereby increasing the level of C3b~inactivating and iC3b~ degradation activity in the subject.
2. A method according to claim 1, wherein the level of C3b-inactivating and iC3b» degradation activity in the subject is increased to a level that exceeds a normal level.
3. A method according to claim 1 or claim 2, wherein the subject is administered with a recombinant virus particle that encapsidates the recombinant viral vector.
4. A method according to claim 3S wherein the recombinant virus particle infects the liver of the subject following administration, resulting in expression of the Factor ls or the fragment or derivative thereof, from the liver.
5. A method according to claim 3 or 4P wherein the recombinant virus particle is a recombinant adeno-associated virus (rAAV) particle encapsidating a rAAV vector,
6. A method according to claim 5, wherein the rAAV particle is pseudotyped to confer liver tropism,
7. A method according to claim 5 or 6, wherein the rAAV particle comprises one or two AAV2 ITRs, or derivatives thereof wherein each derivative AAV2 ITR comprises nucleotide sequence that is at least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, or 99% identical over its entire length with the nucleotide sequence of a naturally occurring AAV2 ITR, and wherein the rAAV particle is pseudotyped with AAV8 capsid protein (rAAV2/8), or AAV2 pseudotyped with AAV9 capsid protein (rAAV2/9), or a derivative thereof comprising amino acid sequence that is at least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%s or 99% identical over its entire length to the amino acid sequence of a naturally occurring AAV8 or AAV9 capsid protein.
8. A method according to any preceding claim, wherein the recombinant virus particle is administered intravenously to the subject.

9. A method according to any preceding cSaim, wherein the recombinant virai vector is a non-integrating, episomal vira! vector.
10. A method according to any preceding claim, wherein the encoded Factor !, or the fragment or derivative thereof, is expressed from a iiver-specific promoter, such as a human a!pha-1-anti~trypsin (hAAT) promoter.
11. A method according to any preceding claim, wherein the leve! of C3b-inacfivating and iC3b-degradation activity in the subject is increased to a level that is up to twice the norma! level.
12. A method according to any preceding claim, wherein the level of C3b-inactivating and iC3b-degradation activity in the subject is increased to a level that is up to 80%, or up to 60%, above the normal level.
13. A method according to any preceding claim, wherein the level of C3b-inactivating and iC3b-degradation activity in the subject is increased to a level that is up to 40%, or up to 20% above the normal level.
14. A method according to any preceding claim, wherein the level of C3b-inactivating and iC3b-degradation activity in the subject is increased to a level that is at least 5%, 10%, 15%, 20%, or 25% above the normal level.
15. A method according to any preceding claim, wherein the level of C3b-inactivating and iC3b-degradation activity in the subject is the level in serum of the subject.
16. A method according to any preceding claim, wherein the normal level of C3b-inactivating and iC3b-degradation activity in the subject is equivalent to that provided by 30-40jug/ml Factor I in serum of the subject.
17. A method according to any preceding claim, wherein the complement-mediated disorder is a disorder associated with over-activity of the complement C3b feedback cycle.
18. A method according to any preceding claim, wherein the complement-mediated disorder is age-related macular degeneration (AMD) (particularly early (dry) AMD, or geographic atrophy), dense deposit disease (DDD), atypical haemolytic uraemic syndrome (aHUS), C3 glomerulopathies, membranoproliferative glomerulonephritis Type 2 (MPGN2), atherosclerosis, chronic cardiovascular disease, Alzheimer's disease, systemic vasculitis,

paroxysmal nocturnal haemoglobinuria (PNH), inflammatory or autoinflammatory diseases of old age, membranoproliferative glomerulonephritis type i (MPGN type I), membranoproliferative glomerulonephritis type III (MPGN type III), Guiliain-Barre syndrome, Henoch-Schonlein purpura, IgA nephropathy, or membranous glomerulonephritis.
19. A method according to claim 18, wherein the subject is at risk of developing AMD,
20. A method according to claim 19, wherein the subject is homozygous or heterozygous susceptible for one or more SNPs associated with AMD.
21. A method according to claim 19 or 20, which further comprises determining whether the subject is at risk of developing AMD.
22. A method according to claim 21, wherein it is determined whether the subject is at risk of developing AMD by determining whether the subject is homozygous or heterozygous susceptible for one or more SNPs associated with AMD.
23. A method according to claim 20 or 22, wherein the level of C3b-inactivating and iC3b-degradation activity in the subject is increased to a level that is at least 10% above the normal level if the subject is heterozygous susceptible for one or more SNPs associated with AMD.
24. A method according to claim 20 or 22, wherein the level of C3b-inactivating and iC3b-degradation activity in the subject is increased to a level that is at least 50% above the norma! level if the subject is homozygous susceptible for one or more SNPs associated with AMD.
25. A method according to any preceding claim, which further comprises determining the level of C3b-inactivating and iC3b-degradation activity in the subject at least a week after the administration, and repeating the administration if the level of activity is found to be at, or below the normal level.
26. A method according to any preceding claim, wherein the Factor! is human Factor I with an amino acid sequence of SEQ ID NO: 2 or 4.
27. A method according to any preceding claim, wherein the fragment or derivative of Factor I is a polypeptide that has at least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, or 99% amino acid identity across its entire length to human Factor I with an amino acid sequence of SEQ ID NO: 2 or 4.
28. A method according to any preceding claim, wherein the subject is a human subject.

29. A recombinant viral vector which comprises nucleic acid encoding Factor I, or a fragment or derivative thereof that retains C3b-inactivating and iC3b-degradation activity.
30. A recombinant viral vector according to claim 29, which is a non-integrating, episomal viral vector.
31. A recombinant viral vector according to claim 29 or 30, wherein the nucleic acid encoding Factor I, or the fragment or derivative thereof, is operably linked to a promoter.
32. A recombinant viral vector according to claim 31, wherein the promoter is a liver-specific promoter, such as a human alpha-1-anti-trypsin (hAAT) promoter.
33. A recombinant viral vector according to any of claims 28 to 31, which is a recombinant adeno-associated virus (rAAV) vector.
34. A recombinant viral vector according to claim 33, which comprises an expression cassette flanked by AAV inverted terminal repeats (ITRs), wherein the expression cassette comprises the nucleic acid encoding Factor I, or the fragment or derivative thereof, operably linked to a promoter and a polyadenylation recognition site.
35. A recombinant viral vector according to claim 34, wherein the ITRs are AAV2 ITRs, or derivatives thereof, wherein each derivative AAV2 ITR comprises nucleotide sequence that is at least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, or 99% identical over its entire length with the nucleotide sequence of a naturally occurring AAV2 ITR.
36. A recombinant virus particle, which comprises a viral capsid encapsidating a recombinant viral vector according to any of claims 29 to 35.
37. A recombinant virus particle according to claim 28 or 29 which is capable of transducing liver cells, particularly hepatocytes.
38. A recombinant virus particle according to claim 36 or 37, which is a rAAV particle,
39. A recombinant virus particle according to claim 38, wherein the rAAV particle is pseudotyped to confer liver tropism.
40. A recombinant virus particle according to claim 38 or 39, wherein the rAAV particle comprises AAV8 or AAV9 capsid protein, or a derivative thereof comprising amino acid sequence that is at least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, or 99% identical over

its entire length to the amino acid sequence of a naturally occurring AAV8 or AAV9 capsid protein.
41. A recombinant virus particle according to claim 40, wherein the rAAV particle comprises one or two AAV2 ITRs, or derivatives thereof wherein each derivative AAV2 ITR comprises nucleotide sequence that is at least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, or 99% identical over its entire length with the nucleotide sequence of a naturally occurring AAV2 ITR.
42. A pharmaceutical composition, which comprises: a recombinant viral vector according to any of claims 29 to 35, or a recombinant virus particle according to any of claims 36 to 41; and a pharmaceutical^ acceptable carrier, excipient, or diluent.
43. A pharmaceutical composition according to claim 42, which is suitable for intravenous administration.
44. A kit, which comprises: a recombinant viral vector according to any of claims 29 to 35, or a recombinant virus particle according to any of claims 36 to 41; and a pharmaceutical^ acceptable carrier, excipient, or diluent.
45. A kit for production of rAAV particles, which comprises: a rAAV vector according to any of claims 33 to 35; and one or more helper plasmids comprising nucleic acid encoding AAV replication and capsid proteins, and genes required for a productive AAV life cycle.
46. A kit according to claim 45, which comprises a first helper plasmid comprising the nucleic acid encoding AAV replication and capsid proteins, and a second helper plasmid comprising the nucleic acid encoding genes required for a productive AAV life cycle.
47. A recombinant viral vector according to any of claims 29 to 35, a recombinant virus particle according to any of claims 36 to 41, or a pharmaceutical composition according to claim 42 or 43, for use as a medicament.
48. A recombinant viral vector according to any of claims 29 to 35, a recombinant virus particle according to any of claims 36 to 41, or a pharmaceutical composition according to claim 41 or 42, for use in the treatment of a complement-mediated disorder.
49. Use of a recombinant viral vector according to any of claims 29 to 35, a recombinant virus particle according to any of claims 36 to 41, or a pharmaceutical composition according

to ciaim 42 or 43, in the manufacture of a medicament for the treatment of a complement-mediated disorder.
50. A vector, particie, or composition according to ciaim 48, or use according to ciaim 48, wherein the complement-mediated disorder is a disorder associated with over-activity of the compiement C3b feedback cycle.
51. A vector, particie, or composition according to claim 50, wherein the complement-mediated disorder is age-reiated macular degeneration (AMD) (particularly early (dry) AMD, or geographic atrophy), dense deposit disease (DDD), atypical haemolytic uraemic syndrome (aHUS), C3 glomerulopathies, membranoproliferative glomerulonephritis Type 2 (MPGN2), atherosclerosis, chronic cardiovascular disease, Alzheimer's disease, systemic vasculitis, paroxysmal nocturnal haemoglobinuria (PNH), inflammatory or autoinflammatorys disease of old age, membranoproliferative glomerulonephritis type I (MPGN type I), membranoproliferative glomerulonephritis type III (MPGN type III), Guillain-Barre syndrome, Henoch-Schonlein purpura, IgA nephropathy, or membranous glomerulonephritis.
52. A vector, particle, or composition according to claim 50, wherein the complement-mediated disorder is age-related macular degeneration (AMD).