You would have thought it was good news: hundreds of thousands of girls are going to be much better protected against the two forms of the human papilloma virus (HPV) that cause 70% of cervical cancers.

But for some, the immunisation programme which launched in Scotland yesterday is dangerous because it "sends the wrong message". Because HPV is sexually transmitted, runs the argument, girls who get the jab are going to think they can sleep with whoever they want and never worry about cervical cancer again.

I'm very suspicious about such "wrong message" arguments. Might not indigestion tablets, for example, send the message that it's OK to eat and drink too much? Could smoke alarms send the message that you needn't worry about fires because they'll be detected soon enough? Given that the human capacity for misinterpretation is infinite, the number of possible wrong-message arguments seems endless.

However, we cannot ignore the fact that, deliberately or not, people see messages that are not explicitly there. That's why marketing and propaganda works. EasyJet's brash white and orange, for example, says "cheap with no frills" more effectively than words to the same effect ever could.

It's also true that, if you change risk factors, you can alter people's incentives - and with them their behaviour. Hence the danger that seatbelts and airbags make drivers feel less vulnerable, and less inclined to drive as safely as they should. So concerns about unstated messages and unintended effects cannot be dismissed. The question, then, is: in any given case, how serious are they?

Let's take an example where it is not too fanciful to think that "wrong messages" were sent. Several HIV/Aids prevention campaigns gave the simple advice: always carry a condom. They worked, to the extent that, for my generation, having a condom in your wallet was a sign of being a mature, sensible adult. But surely this did to some extent normalise promiscuity, by "sending the message" that ordinary, healthy people like you have unexpected sex with strangers all the time.

Yet to conclude that the promotion of condom use was wrong is premature. Most would agree that the risks associated with more promiscuity were not as grave as the risks of less promiscuity with fewer precautions. Casual sex was not exactly rare before Aids came on the scene, so it's not as though the green light was being given to a hitherto taboo activity.

What's more, other campaigns have shown that you can promote safe sex without encouraging casual sex. For instance, global health campaigns, particularly in Africa, have promoted the "ABC" of abstinence, being faithful and using condoms, in that order of priority.

These lessons apply just as well to the cervical cancer vaccination programme. First, and foremost, as long as it prevents significantly more life-threatening disease than it might inadvertently spread, it makes sense to do it.

Furthermore, even if it is true that some girls might see the jab as a kind of tacit initiation into the age of sexual activity, the idea that this is going to be the critical factor in their decisions is somewhat fanciful. There are many more alluring incitements for teenagers to have sex.

If we're worried that girls might misread the signals, the simplest thing to do is make sure they're clearer. When a girl is given the jab, she needs to be told that it doesn't guarantee protection; that there are plenty of other nasty STDs out there as well; and that only an idiot would think the jab was a licence to sleep around. With the right overt messages attached, the programme could be part of sensible sex education.

We are right to stop and think about the ways in which advice and campaigns may alter people's behaviour or be interpreted in ways that are contrary to what we intend. More often than not, however, the solution is not to stop but simply to counter the feared message with a very clear, overt one. Sometimes actions only speak louder than words when you let them.