If a seat becomes vacant between elections, List PR systems often simply fill it with the next candidate on the list of the party of the former representative, thus eliminating the need to hold another election. However, plurality/majority systems often have provisions for filling vacant seats through a by-election. When other systems are in use, either approach may be possible: under STV, the Republic of Ireland holds by-elections for vacant seats in the legislature, but Australia does not do so for Senate vacancies. It is also possible to avoid by-elections by electing alternates at the same time as the ordinary representatives, as is done for example in Bolivia. By-elections are smaller and therefore less costly than normal elections, but in some countries they will nevertheless put a significant burden on the budget, and seats are sometimes left vacant for long periods because of a lack of capacity to arrange by-elections. This is an especially salient problem in some countries in Southern Africa where the HIV/AIDS epidemic often leads to a large number of vacant seats between elections. In some circumstances, by-elections can have a wider political impact than merely replacing individual members, and are seen to act as a mid-term test of the performance of the government. In addition, if the number of vacancies to be filled during a parliamentary term is significant, this can lead to a change in the composition of the legislature and an altered power base for the government.
