Pokémon GO has started rolling out update version 0.421.1 for iOS and Android devices, delivering the latest client update for the mobile game. While no official patch notes have been released for the new version, the update is expected to focus on general bug fixes, performance improvements and stability enhancements as the game continues to prepare for upcoming events and future content. As with previous Pokémon GO app updates, the rollout is being released gradually, meaning availability may vary by device and region before version 0.421.1 reaches all players.
Source: Official Pokémon GO support
Nintendo & Pokémon Blog 