A vaccine which triggers the immune system to attack the most common type of lung cancer has shown promise in early clinical trials, say researchers.
Tests on 148 patients, reported in the Lancet Oncology, showed that adding the vaccine to chemotherapy slowed the cancer's progression.
However, its effect on overall survival was limited and further trials are now needed.
Cancer Research UK said there were many unanswered questions.
Vaccines for cancer use the same principles as vaccines against infection - training the body's own immune system. However, instead of protecting against measles or seasonal flu, these vaccines attack tumours growing in the body.
The idea is that when a cell becomes cancerous and divides uncontrollably, its starts to look different. Proteins on the surface of the cells change and the immune system can be trained to spot these changes.
Researchers at the University of Strasbourg used a vaccine called TG4010. It is a modified pox virus, distantly related to smallpox, which has been genetically modified to make a "cancerous" surface protein.
Patients with advanced non-small-cell lung cancer took part in the trial. All were given standard chemotherapy treatment, half were also infected with the virus.
Six months later, the illness was more likely to be stable in vaccinated patients than in those just taking chemotherapy drugs. Six month "progression free survival" was 43% for vaccinated patients and 35% for those on chemotherapy.
However average survival was 10.7 months in vaccinated patients, only marginally higher than the 10.3 months in chemotherapy patients.
Prof Peter Johnson, chief clinician at Cancer Research UK, said: "There's a lot of interest in harnessing the power of the immune system to treat cancer. This early-stage study shows that combining a vaccine with chemotherapy is possible, and may have some benefits for some people with lung cancer.
"But this study leaves a lot of unanswered questions - further research is needed to see whether the vaccine will actually improve survival for lung cancer patients."