Are there differences in transmission?

It has been observed that certain subtypes/CRFs are predominantly associated with specific modes of transmission. In particular, subtype B is spread mostly by homosexual contact and intravenous drug use (essentially via blood), while subtype C and CRF A/E tend to fuel heterosexual epidemics (via a mucosal route).

Whether there are biological causes for the observed differences in transmission routes remains the subject of debate. Some scientists, such as Dr Max Essex of Harvard, believe such causes do exist. Among their claims are that subtype C and CRF A/E are transmitted much more efficiently during heterosexual sex than subtype B.^{9 10} However, this theory has not been conclusively proven.^{11 12}

More recent studies have looked for variation between subtypes in rates of mother-to-child transmission. One of these found that such transmission is more common with subtype D than subtype A.¹³ Another reached the opposite conclusion (A worse than D), and also found that subtype C was more often transmitted that subtype D.¹⁴ A third study concluded that subtype C is more transmissible than either D or A.¹⁵ Other researchers have found no association between subtype and rates of mother-to-child transmission.^{16 17 18 19}