The number needed to treat (NNT) and population extensions of the NNT describe the population outcomes of disease treatment, usually for chronic diseases. We have developed similar measures for vaccine preventable diseases based on the number needed to vaccinate. These measures quantify the number of people, or the number of vaccine doses, needed to prevent one event due to disease per year and allow the calculation of the vaccine cost to prevent one event due to disease per year. We used this method to compare influenza and pneumococcal vaccine programmes for people aged 65 years or more. We calculated the vaccine cost (dollars AUD) to prevent a case of disease, a hospitalisation or a death due to influenza or invasive pneumococcal disease each year in this age group. For influenza, the vaccine costs were dollars 598 to prevent one case per year, dollars 10787 per hospitalisation prevented and dollars 74801 per death prevented each year. We assumed all cases of invasive pneumococcal disease were hospitalised so the vaccine cost of dollars 11494 per case prevented was the same as the vaccine cost per hospitalisation prevented, while the vaccine cost per death prevented each year was dollars 49972. These results suggest the vaccine costs for the prevention of one hospitalisation or one death each year due to invasive pneumococcal disease among the elderly in Australia would be similar to the costs for the prevention of one hospitalisation or death due to influenza.