Over the past 18 months, MSF has vaccinated over 500,000 children in a continuing campaign against measles in some of the most inaccessible areas of the Democratic Republic of Congo (DRC). The next stage of the campaign will cover over 100,000 children in hundreds of villages across the health zone of Basankusu; an area about half the size of Belgium, but only accessible by pirogue or, at best, motorbike. Swedish nurse Jessica Nestrell is coordinating the vaccination, and over the next six months she will be sending regular dispatches from the field.

We've finally made it back to the main MSF base in Mbandaka to work on the information sessions for the 147 vaccination teams and the planning for the medical materials we will need for this vaccination effort. We had started to head back from Basankuso yesterday, at 5:30am, when the sun was rising and we hoped to make it here before it set. Unfortunately, that didn't quite happen. We didn't arrive at the port here until after dark, having spent the full day travelling, under the merciless sun, down the Ikilemba, one of the little tributaries of the larger Congo river.

We had gotten up so early that we didn't have any breakfast before leaving. Instead I bought 12 bananas and planned to buy bread on the market before heading off. In the end we didn't have time to get the bread and I left all the bananas on the kitchen table. So much for my plans.

After searching my bag for something edible (13 hours without any food is a lot) I found some chewing gum. We had just one each for breakfast, saving the rest for lunch and dinner. Fortunately, mid-afternoon we did a 10 minute stop with the pirogue at an old Catholic mission station in the middle of the jungle.

This little mission station is little more than a small village with a big church in the centre. However it is a very important toilet opportunity and not to be missed since we could only make one stop during the whole day. Initially I had thought it would be possible to just stop anywhere - afterall, this is a jungle. But you cannot, since the rain forest is so dense that it is impossible to get into it. Instead, every stop has to be well planned. Oh well. We managed to buy some bananas and bread at the mission station so we could finally eat something.

I was almost ready to dive in myself to save the chicken. In the end, I decided not to because of the crocodiles.

Some of the people on the pirogue were taking animals to Mbandaka. When we got on, there were five chickens and a pig, lying exposed in the sun, that I covered with plastic sheeting for protection. Sadly one of the chickens fell overboard. It was in a box in the other pirogue and somehow it got up on the log used to connect the two pirogues together. When we tried to catch her, she fell into the water.

I thought for sure we would turn the boat around, because her feet were tied together. I was almost ready to dive in myself to save her. In the end, I decided not to because of the crocodiles. Since we shouted to the people in the village that we passed that there was a chicken in the water, she was probably on someone's dinner-table the same tonight.

Throughout the vaccination campaign, we need to create a 'cold chain' where the vaccinations are constantly kept at low temperatures before use. If the cold chain breaks, the vaccinations will be useless. It all needs to be meticulously planned. Or at least as meticulously as the realities of Basankusu allow.

The last few days in Basankusu were jammed with work, trying to finish the planning and structuring all the data we collected from the field. We have gone through all the different villages in the area. There are hundreds. Some of them are so inaccessible that it is difficult to find out if people still live there or if they have moved. Many people live by the small little rivers in the forest, only accessible by paddling on the small local pirogues - sometimes for days.

Each vaccination team will be made up of five people. We have made lists of how many teams are needed and we are still trying to decide where we place the site to stock the vaccines and where to keep the preparation sites.

There will be 100,000 vaccination vials used in this effort and keeping the vials healthy is essential. For this we need to create a 'cold chain', where the vaccinations are constantly kept at low temperatures before use. If the cold chain breaks, the vaccinations will be useless.

The preparation site is really the heart of the cold chain (needed to constantly keep the vaccines between 2Ã?° and 8Ã?°C) and that site is then used to supply all the stockage sites.

That is where we will keep the generators used for running the refrigerators. It has to be sort of a spider in the middle of his net. In the stockage sites we keep the vaccines in large cool boxes and go back and forth from there to the preparation site to supply the stockage site with cold ice packs. It all needs to be meticulously planned. Or at least as meticulously as the realities of Basankusu allow.

When the vaccination campaign starts, we will have split the area in three different pools. The last pool, Djombo, is going to be the toughest since we have to transport everything by river and set everything up from one preparation site.

When the vaccination campaign starts, we will have split the area in three different pools. Pool One is Waka, situated south east of Basankuso. Pool Two is Bokakata to the south and finally Djombo, Pool Three, is to the north east. The easiest will be Waka, so we have decided to start with that. The last pool, Djombo, is going to be the toughest since we have to transport everything by river and set everything up from one preparation site.

To complicate the matter, two thirds of the people in Djombo live in the bush and so are only accessible by foot or pirogue. The third that will be reached by motorcycle seems to be easier, although we have had reports that some stretches of the road are in a miserable condition and it will be horribly difficult to come through with the heavy cool boxes on the back of the motorcycles.

Plus we have not had the chance to visit the area. This is not ideal although, with the help of some people from the area, we have managed to complete the very inaccurate map.

Today it is Sunday and a day of rest, but since we didn't really do much yesterday we have decided to work a bit anyway. There is page after page of information on all the health centres and the villages. And all of this has to be prepared for the report due in Kinshasa by the end of the week.