Do you know the reinfection was a different strain? Or did the immunity to the first strain weaken?
A patient must present a infection and the particular virus must be sequenced, and then present with an infection later on where the virus is again sequenced and identified as sufficiently different from the first sample. Otherwise it cannot be deemed a re-infection.
If these small differences in the genomic sequence are already a different strand, I don't know, not an expert. But all 5 (?) cases known so far had a different sequence identified the second time.
> The genomes of the patient’s virus samples from April and June displayed significant genetic differences between them, suggesting that the patient was infected twice by 2 distinct COVID-19 infections.
I’d read of known second infections that are from different strains, but scientists also question lasting immunity from COVID-19 infection because other coronaviruses are seasonal.
It was possible to positively identify a reinfection because the two infections were due to different strains. That was just the means of positively and objectively identify a reinfection. However, nothing was asserted regarding if the patient was immune to the first strain the first time around. In fact, it seems that the hallmark of reinfections is that they are far more critical than first infections. This also means that the first infections is far milder than the second one. Well, the thing about the immune system is that it is able to fight mild infections without developing immunity if they are too mild to trigger a full response from the immune system.