Disease progression after infection.
Disease progression from infection to AIDS follows a set pattern in most infected individuals, although the rate of disease progression varies from one person to the other.
- Rapid progressors take 3–6 years
- Average progressors take 8 years
- Slow progressors take 15 years
There is a small group of people who are classified as non-progressors. The slow progression of these individuals is thought to be as a result of genetically inherited factors.
HIV attacks cells in the immune system (which have a certain type of receptor called CD4+). When an individual is first infected, the initial stage of the disease is characterised by the rapid spread of the virus throughout the body. There is also an intensive phase of viral replication resulting in high quantities of the virus being present, and a corresponding decrease in CD4+ count.
As the body’s immune responses begin to take effect and antibodies are produced, the amount of virus declines and the immune system recovers. The period between infection and the production of antibodies is known as the window period (it is during this interval of time that an infected person may test negative for HIV). It is also the reason why people who test negative should repeat the test after three months in case they have been recently infected.
The control of HIV by the immune system then continues over a period known as the ‘clinical latency’. During this time viral replication is contained, although there is a steady weakening of the immune system, indicated by a decrease in the CD4+ count by anywhere between 40–80 cells per year.
The rate of disease progression to AIDS is dependent on a variety of factors, one of which is the viral ‘set point’. This is the level of HIV in the blood established after initial infection. A person with a low viral set point (in other words a small amount of HIV in the blood) is less likely to develop advanced disease at a rapid rate. A person with a high viral set point (in other words a high level of virus in the blood) is likely to show more rapid disease progression. Other factors that may influence the person’s progression to the advanced stages of HIV may include nutrition, stress levels, and other lifestyle issues.
Once the CD4+ count falls below 350, there is an increased likelihood that certain of the opportunistic infections associated with advanced disease will manifest, and the types and numbers of infections may increase as the patient’s CD4+ count drops.
It is on this basis that the indicator for starting treatment has been established in many countries as a CD4+ count of 350 or less, or when certain opportunistic infections become evident—whichever is the earlier.
Once treatment is started then the whole process can be reversed and there is normally an increase in CD4+ count and a corresponding decrease in viral load.