Thank the sneezer instead. If they wonder why you thanked them you can thank them for the aerosol of bacteria and crud they sent your way.

In east Asia it's customary for sick people to wear masks to avoid spreading disease via coughing or sneezing. A practice that ought to be adopted everywhere.

I know I'm promoting a passive-aggressive strategy which is also social corrosive, but it's a way of spreading (!) the meme. And don't say "Gesundheit" because you think "bless you" is religious -- saying Gesundheit is simply non-denominationally superstitious.