I am the leader of a large country. My position and tenure as leader is secure.

My objective is to ensure that my country becomes globally dominant. My timeframe for achieving my objective can be measured in decades, rather than years. I have taken steps to project influence and power at a regional and, increasingly, at a global level.

Economic development within my country depends on constant high levels of production and growth. I realise that future demographic shifts in the population will impede growth. In particular, the ageing of the population will create the need for increased social security nets, requiring higher public investment and revenue generation among younger workers. This situation is exacerbated by previous shortsighted measures aimed at limiting population growth.

I therefore need to determine a way in which I can reverse this demographic shift. Increased immigration is not feasible. Increasing the birth rate is difficult as urbanisation increases. I decide therefore that I just reduce the number of elderly people.

i instruct my scientists to create and leak a virus that disproportionately affects elderly people. The death rate must be significant, but not too deadly so as to allow it to burn out quickly. Asymptomatic transmission makes it virtually impossible to control and allows for it to spread quickly and efficiently within the population. A neat side effect is that it also destabilises my most aggressive geo political rivals. By eventually releasing the vaccine (pre-prepared) I will elevate my nations standing in the world and expand influence through soft power initiatives eg free mass vaccinations. 

Implausible?