Hoe effective is elisa hiv test after 8 months?

It is not possible to know whether 98% or 99% or 99.999% of people who have been infected for eight months show up as seropositive in an ELISA. There just are not enough people with a single exposure, who then get tested at 7, 8, and 9 months or more to know exactly how long after infection they seroconverted.

However, the data that has been collected shows that seroconversion more than 6 months after a single exposure is rare. Almost everyone who is infected produces enough antibodies to be detected in the ELISA within 6 months of infection.

Ciesielski CA, Metler RP.
Duration of time between exposure and seroconversion in healthcare workers with occupationally acquired infection with human immunodeficiency virus.
Am J Med. 1997 May 19;102(5B):115-6.
PMID: 9845512
Through December 1994, 41 healthcare workers with a documented seroconversion to human immunodeficiency virus (HIV) in temporal association to an occupational exposure were reported to the Centers for Disease Control and Prevention (CDC). Each tested positive for HIV antibodies within 12 months of the occupational exposure. Two (5%) of the 41 tested negative for HIV antibodies >6 months following the occupational exposure but were seropositive within 12 months of the injury. Both denied any subsequent exposures to HIV after the initial exposure, and in one case genetic sequencing confirmed the source of the infection. Four of the healthcare workers took postexposure zidovudine prophylaxis; each reported an acute retroviral syndrome within 6 weeks of their exposure, and each of the four seroconverted to HIV within 6 months of the exposure. Our data suggest that zidovudine prophylaxis does not delay the development of HIV antibodies beyond 6 months. Because many of the healthcare workers had follow-up testing at irregular intervals, with long periods between tests, it was not possible to define precisely when seroconversion occurred. However, our findings are compatible with previously published estimates that 95% of infected persons will develop HIV antibodies within 6 months of infection.