By a journey to Comore Islands on less than 1 month recommend protection against the following infections:

Diphtheria
Diphtheria is a serious throat infection, which infects from person-to-person through the air. The vaccination should be less than 10 years old otherwise a booster is needed.
Read more about diphteria here.

Malaria
There is considerable risk of malaria and prophylaxis is recommended. The choice is between Malarone, Doxycycline or Lariam. Protection against mosquito bites will reduce the risk further. 
The distribution of malaria can be seen at the map below.
Read more about malaria here.

The information on malaria risk in individual countries are based on the sources found here

Hepatitis A (infectious hepatitis)
Infectious hepatitis infects through contaminated food and water. Vaccination consists of two injections about 12 months apart, which protects for up to 25 years. The first vaccination protects for 12 months. 
Read more about hepatitis A here.
The hepatitis A vaccine can be combined with the vaccine against hepatitis B

HIV in Africa
109 Danes were infected with HIV in Africa between 1991 and 99. Africa is the continent with most HIV infected people. Overall about 8% of the population is infected with infection rates reaching up to 80% in risk groups like prostitutes.
Read more about HIV here.

Tetanus
Tetanus is a complication to wounds contaminated by soil. If there has been a vaccination within the past 10 years it is not necessary to give a booster in case of wounds and accidents. Tetanus vaccinations are usually given in combination with diphtheria vaccine. If you are previously vaccinated, the vaccine can be given right up to departure.

Recommended vaccinations Starting Diphtheria 1 day Malaria 1-7 days Hepatitis A (infectious hepatitis) 1 week HIV in Africa Tetanus 1 day 1-7 days When to start vaccinations before leaving::