Inverted covariate effects for mutated 2nd vs 1st wave Covid-19: high temperature spread biased for young

(1) Background Here, we characterize COVID-19 2 nd waves, following a study presenting negative associations between 1 st wave COVID-19 spread parameters and temperature; (2) Methods Visual examinations of daily increase in confirmed COVID-19 cases in 124 countries, determined 1 st and 2 nd waves in 28 countries; (3) Results 1 st wave spread rate increases with country mean elevation, temperature, time since wave onset, and median age. Spread rates decrease above 1000m, indicating high UV decrease spread rate. For 2nd waves, associations are opposite: viruses adapted to high temperature and to infect young populations. Earliest 2 nd waves started April 5-7 at mutagenic high elevations (Armenia, Algeria). 2 nd waves occurred also at warm-to-cold season transition (Argentina, Chile). Spread decreases in most (77%) countries. Death-to-total case ratios decrease during the 2 nd wave, also when comparing with the same period for countries where the 1 st wave is ongoing. In countries with late 1 st wave onset, spread rates fit better 2 nd than 1 st wave-temperature patterns; In countries with ageing populations (examples: Japan, Sweden, Ukraine), 2 nd waves only adapted to spread at higher temperatures, not to infect children. (4) Conclusions 1 st wave viruses evolved towards lower spread and mortality. 2 nd wave mutant COVID-19 strain(s) adapted to higher temperature, infecting children and replace (also in cold conditions) 1 st wave COVID-19 strains. Counterintuitively, low spread strains replace high spread strains, rendering prognostics and extrapolations uncertain.