The phrase "almost lethal" is correct and usable in written English.
It can be used to describe something that is very close to being deadly or fatal, but not quite there.
Example: "The poison was almost lethal, causing severe symptoms but ultimately not resulting in death."
Alternatives: "nearly fatal" or "close to deadly".
Richardson fondly recalls the uppity antics of Prime Minister Heath's security men, remembers the courtyard choked with the chauffeured cars of Mrs. Cowles's glittering guests, and his own subtle, almost lethal handling of a secretary once reluctant to admit him to the sacred precincts.
A woman in the crowd is wearing a lit-up lampshade and swaying her heavy head back and forth; next to her is a man in antlers, and another whose cape has spikes on each shoulder which look almost lethal in the tightly packed crowd.
Speaking of keepers, Boubacar Barry has just played a blinder at the other end to deny Zambia a goal after the Katongo brothers combined to (almost) lethal effect.
"It would be almost lethal to put efforts into other products," Cannie says.
The film's true subject is explicit from the opening quote: "The rush of battle is a potent and almost lethal addiction, for war is a drug".
For example, the laboratory HGT data contain only absolutely nontransferable genes, which are lethal or almost lethal to the recipient when transferred.
Even by the standards of the Philippines, which is prone to typhoons as well as earthquakes and volcanic eruptions, Haiyan is a natural disaster of epic proportions – and almost certainly its most lethal to date.
WASHINGTON — Until the anthrax attacks of 2001, Bruce E. Ivins was one of just a few dozen American bioterrorism researchers working with the most lethal biological pathogens, almost all at high-security military laboratories.
We demonstrated that expression of RNA interference (RNAi) for the rt gene and the tw mutant was almost completely lethal and semi-lethal, respectively.
With DDT as his weapon, Soper almost saved the world from one of its most lethal afflictions.
The growing of flies with ubiquitous expression RNAi for the rt gene at a higher temperature resulted in higher lethality rates, and knockdown at 28°C was almost entirely lethal (Table 8).
