She is said to have intentionally added insulin to the infant’s intravenous feed bag, less than 24 hours after she allegedly murdered his twin brother on the neo-natal unit of the Countess of Chester Hospital.
Jurors at Manchester Crown Court were told Child F’s heart rate surged and his blood sugars dropped dangerously low after the feed started shortly after midnight on August 4 2015.
His parents were “keen” to have him moved from the hospital earlier in the day after the death of their son, Child E, on the previous night shift but were told no transport was available.
Following the end of the night shift, in which Letby is said to have poisoned Child F, the defendant messaged a colleague, who cannot be named for legal reasons, about the infant’s low blood sugars.
She wrote: “Something is not right … he’s a worry.”
Letby, 32, messaged another colleague, Minna Lappalainen, later on the early evening of August 5.
She asked: “Are you going to salsa tonite?”
Her colleague replied: “Should do really as I have not been for ages. Meet at TF 2020?”
Letby responded with an OK emoji.
She added: “Need to try and find some post nites energy.”
Ms Lappalainen answered: “Don’t have to stay late.”
Letby responded: “Hasta luego.”
The nursing colleague, who cannot be identified, later messaged Letby from the hospital to say Child F was “a bit more stable” and medics were “trying various tests to try to find answers”.
Letby replied: “Oh dear. Thanks for letting me know.”
Her colleague said: “He’s defo better tho. Looks well, handling fine.”
Letby later asked: “Wonder if he has an endocrine problem then. Hope they can get to bottom of it.
“On way home from salsa with Minna.
“Feel better now I’ve been out.”
The court heard the defendant had a house-viewing appointment the following day.
Child F went on to recover in the days that followed the alleged sabotage on the night shift.
His elder brother, Child E, born a minute earlier, is alleged to have been killed with a fatal injection of air into his bloodstream.
Jurors have heard Letby is also said to have attacked another set of twins, her first two alleged victims, in June 2015.
She is said to have murdered Child A by administering air into his bloodstream and then attempted to murder his sister, Child B, on the following night shift, by using the same method.
Letby, originally from Hereford, denies the murders of seven babies and the attempted murders of 10 others between June 2015 and June 2016.
The trial continues on Wednesday.