I thought I would attempt to answer my own question. The only other answer currently (by Larian LeQuella), while a helpful comment, I feel it doesn't answer my original question.
To begin, we need to put the results into context.
Fact: Hand-washing using tap water alone will reduce the amount of bacteria and viruses on hands (they might not be actually killed, but no longer on your hands).
Ansari et al. (1989) performed an experiment where a fecal solution was placed on participants' hands and subsequently washed off using one of a range of soaps. They found a reduction in around 83% of the human rotavirus and 90% of the E. coli.
Shojaeia et al. (2006) randomly selected 150 food handlers in Iran, and instructed them to wash and scrub their hands with sterile water. Before the intervention, around 73% were contaminated with bacteria (primarily with Staphylococcus aureus or E. coli), and afterwards 32%.
Fact: More bacteria and viruses are removed from hands by hand washing by using some type of soap (not necessarily antibacterial).
Ansari et al. (1989) (op. cit.) found that certain agents (e.g. 70% isopropanol) can increase the reduction in E. coli and human rotavirus to around 98%. However, this was not true of all possible hand-washing agents.
Mbithi et al. (1993) also conducted a experiment consisting of placing a fecal solution on participants' hands that was subsequently washed off using one of a range of soaps. In this case, the hepatitis A virus (HAV) and poliovirus type 1 (PV) was considered. They found that tap water gave a reduction of HAV of around 80% and PV of around 85%. The reduction increased to around 88-92% for HAV and 90-98% for PV for most soaps tested.
[There will be zillions of additional examples to the above.]
A large survey was conducted by Aiello et al. (2007) which considered the efficacy of triclosan (one of the common active ingredients used in antibacterial hand soap) found that, overall, previous research has indicated no significant additional benefit vs. ordinary hand soap. They write:
Soaps containing triclosan within the range of concentrations commonly
used in the community setting (0.1%–0.45% wt/vol) were no more
effective than plain soap at preventing infectious illness symptoms
and reducing bacterial levels on the hands.
This is probably not the last word on the matter, e.g. Fischler et al. (2007) claimed a significant difference in the transmission and acquisition of E. coli and Shigella flexneri after using an antibacterial soap. It is also plausible that antibacterial agents other than triclosan (where the focus has been) provide a health benefit (for example, methylchloroisothiazolinone/methylisothiazolinone). In fact, the Aiello et al. (op. cit.) paper itself indicates a non-negligible effect from >1% triclosan. Kimberly-Clark give a 99.9% or more bacterial killing efficacy for a range of bacteria in a 2003 in vitro experiment of their product; obtainable via Googling "kimberly-clark killing efficacy".
(Side note: There is also a suspicion that the use of triclosan will give rise to triclosan resistance or cross-resistance to antibiotics (e.g. by Levy (2001), Aielloa and Larson (2003), Yazdankhah et al. (2006)). Although, Cole et al. (2003) and Weber and Rutala (2006) claim otherwise in the case of antibiotic resistance.)
This leads to the original question: Which common diseases would likely be more affected by antibacterial hand soaps (and other antibacterial products) than their non-antibacterial counterpart? I.e. if we wanted to test the claim that a certain antibacterial hand soap has some positive effect (vs. a non-antibacterial version), what would be some common diseases that we could test for, and expect to find a non-negligible result?
I offer the following candidate for a bacterial disease whose effect should be noticeably reduced by the use of antibacterial hand soaps:
- Staphylococcus aureus is a bacterial species that causes a range of noticeable illnesses and can be transferred by skin-to-skin contact and contact with contaminated objects. We can thus reasonably expect it to be affected by an effective antibacterial hand soap. In fact, washing hands is recommended for reducing the prevalence of S. aureus, along with zillions of other diseases (by e.g. Better health channel). This is one of the bacteria that Kimberly-Clark give a 99.99% kill efficacy of their product in vitro.
Although, not directly related to hand-washing, Brady et al. (1990) used a 1% triclosan preparation and Zafar et al. (1995) used 0.3% triclosan to control methicillin-resistant Staphylococcus aureus.
The single additional measure of changing handwashing and bathing soap to a preparation containing 0.3% triclosan (Bacti-Stat) was associated with the immediate termination of the acute phase of the MRSA outbreak. -- Zafar et al.
Similarly, a 1.5% triclocarban bath has been used to treat atopic detmatitis Breneman et al. (2000):
The antimicrobial soap regimen caused significantly greater
improvement in the severity and extent of skin lesions than the
placebo soap regimen, which correlated with reductions both in S
aureus in patients with positive cultures at baseline and in total
The above results suggest that antibacterial soaps could have a beneficial effect against S. aureus (more than a non-antibacterial counterpart). However, it is unclear as to how much of this effect implies a comparable effect for hand washing alone (and in an ordinary household setting). Moreover, there are many different antibacterial hand soaps and other products, whose active ingredients might be at different dosage, all of which would further affect the outcomes.