I'm wondering about what's meant when someone says a vaccine is, say, 97% effective. Do they mean that the vaccine is effective for 97% of all people, or that for all people it is only effective 97% of the time? This seems like an especially important distinction in the context of sexual health. Say there is a vaccine developed for HIV (or HSV, doesn't matter). If the vaccine is 97% effective for all people, then this would mean that there is still a 3% chance of transmission per exposure. This could be a big problem for people who have large amounts of sex. On the other hand, if the vaccine is completely effective for 97% of the population, and ineffective for 3% of the population, then even for people who have frequent amounts of sex, the risk of transmission would be fixed at 3%.

Am I making any sense?

Your question not only makes sense; it's very insightful. Vaccine effectiveness has both meanings you cite. But the situation is even more complex. Here comes one of my occasional blog-like replies to provide education to all readers and might be useful in replying to future questions on the topic.

Tetanus vaccine is 100% protective in both senses:  no risk of tetanus in people with dirty wounds, and probably every vaccinated person experiences this complete protection. That is, 100% biologically effective in preventing tetanus, with 100% of vaccinated persons being protected. Influenza vaccines are only about 60% effective in preventing infection in an exposed person, and probably 80-90% of vaccinated persons experience this level of protection. Here's where it gets more complex. Some vaccines aren't all that great at preventing infection, but highly effective in reducing severity if it happens. Influenza and the mRNA COVID-19 vaccines are like this; for covid this has been covered extensively in the media. Probably almost everyone vaccinated gets some level of protection. In that sense it's 100% effective:  almost all vaccinated persons are protected against illness severe enough to result in hospitalization or death. 

For still more complexity, consider reduced but still substantial effectiveness if 3 vaccine doses are ideal but some people only get one or two doses, which is the case for the main STD vaccine -- Gardasil, the main HPV vaccine in the US. Three doses over six months is 100% protective against infection with the 9 HPV types that cause 90% of genital warts and various cancers. Nearly 100% of people who receive the vaccine have that excellent protection. So by both measures it's among the most effective vaccines ever produced. However, the vaccine doesn't cover the other ~110 types of genital (sexually transmissible) HPV strains, and sometimes these cause warts or cancer. In other words, if benefit is measured in preventing HPV strains the vaccine is designed to prevent, effectiveness is 100%; but its effectiveness in preventing genital and HPV related cancers is only 90%. But effectiveness is lower in those who don't get all three doses, and especially in those who receive only one dose.

Continuing on STDs, recent research has shown that the meningitis vaccine now recommended for young people in group situations like the military, college students in dorms, etc not only prevents infection with Neisseria meningitidis -- the main cause of bacterial meningitis -- but also reduces the frequency of gonorrhea by 50%. (Gonorrhea is caused by Neisseria gonorrhoeae. As the similar names imply, N. gonorrhoeae and N. meningitidis are closely related.) A new vaccine against gonorrhea itself has been developed and is in clinical trials. Nobody expects it to be 100% effective in preventing gonorrhea or to be protective in 100% of those who receive it. Whether it will reduce disease complications and severity isn't yet known. But these less than complete protection levels do not mean the vaccine will be useless. If it works as hoped, it have tremendous benefit worldwide.

So your closing calculation is correct:  A hypothetical vaccine that is 100% effective in preventing infection might offer that protection to only 97% of those who receive it, i.e. 3% unprotected. However, even these may benefit greatly if it's a vaccine that reduces severity without preventing all infections.

Does this make sense? Let me know if anything isn't clear.

HHH, MD

However, I would like to address a comment you made about my concerns surrounding herpetic whitlow. It is great to hear that there isn't a risk for asymptomatic shedding, but would you be willing to humor me about why this is? From a lay perspective (i.e., mine) it seems like sweat could be a vector for transmission since sweat glands are under all those layers of skin.

Also, I have heard elsewhere that we can assume HW doesn't shed asymptomatically because there isn't any evidence that suggests otherwise. However, I worry that if fingers did shed asymptomatically, it would be difficult to gather evidence demonstrating that they do. My reason for thinking this is that the vast majority of people who have herpetic whitlow also have either oral herpes or genital herpes (this is based on the only large-scale study of HW I could find: "Herpes Simplex Virus Infection of the Hand: A Profile of 79 Cases" by Gill et al.)

I looked back at your discussion with Terri and agree entirely with her comments. To my knowledge there has never been any research on asymptomatic shedding from the site of herpes whitlow; at that level, I suppose we can't be dogmatic about it. However, genital and oral asymptomatic shedding of HSV is largely from mucosa -- that is, moist and internal surfaces. While shedding can be detected on dry skin (say penis, scrotum, labia major), in absence of lesions some of this may reflect contamination from mucosal surfaces. (Terri gave the example that people who have had nearby but non genital recurrent herpes, e.g. on the buttock, probably do not infect partners from those sites.) I agree there is no reason to expect shedding from the dry, cornified skin of the fingers. Further, busy STD clinics rarely if ever see patients with genital or anal herpes who had not direct genital or oral-genital exposures. There are plenty of people with past herpes whitlow who must have fingered their partners, but I am unaware of any cases of genital or anal HSV whose only exposure was fingering or other hand-genital contact. I'm pretty sure this also Terri's experience -- and she may have seen more patients with genital herpes than any other single health care provider anywhere. (I'll ask her to take a glance at this statement and perhaps confirm it.)

Like Terri, I do not think you need be at all concerned about infecting partners by fingering or other hand-genital contact, assuming you're not having a recurrent whitlow at the time.

Apologies again for the long delay in this reply.

Do your best to let it go! 

Thanks for the thanks. I hope we've helped. Best wishes and stay safe.