english: The second challenge we had was -- in contrast to the smallpox vaccine, which was very stable, and a single dose protected you for life -- the polio vaccine is incredibly fragile.
non_english: İkinci engel ise, çok kararlı olan ve tek dozun hayatınızı kurtardığı çiçek aşısının tersine çocuk felci aşısı inanılmaz derecede kırılgan.