For the successful development of a vaccine to be used against a pathogen, it is necessary that

the surface antigens of the pathogen stay the same

Explanation:

A vaccine is a biological composition that proffers active acquired immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and it is produced in that respect to weakened or killed forms of the microbe, its toxins, or its existence on the  surface proteins. Like we all know that Pathogens can be  bacterium, virus, or other microorganism that are capable of  causing diseases and also Antigens are toxin or other foreign substance which prompt an immune response in the body, especially the production of antibodies.

Now for  the successful development of a vaccine to be used against a pathogen, it is necessary that the surface antigens of the pathogen stay the same because this will help in production of a vaccine that can help to combat the effect of the pathogens.

answer; both surfaces of the membrane are nonpermeable