Reff

Way back at the beginning of the pandemic there was a lot of discussion about the reproduction number of the virus which I dealt with under the heading Ro.  The problem was to translate the maths symbol R (0) to something everyone could say and write so R naught was one possibility.  Others were RO or Ro (pronounced /roh/).  Just as a refresher an RO of 1 meant the disease was still there but stable. An RO of less than one meant that for every diseased person there was less than one other person catching it, so the virus was on the way out. If the RO was greater than 1 then for every diseased person there was more than one person catching it, so the virus was spreading.

 Now we have the further subtlety of the Reff (effective reproduction number), also written Re or R (t). This number takes into account the fact that we now have some part of the population vaccinated or now immune, and the community is taking precautions such as social distancing and mask wearing. Sometimes we expand the term to Reff number just because it seems more helpful. The Reffs are to be interpreted in the same way as the RO.