What is the difference between the adjectives novel and new?

Solution 1:

Yes, as Weather Vane says, 'novel virus' forces such a specifying definition on 'novel' that it is perhaps better regarded here as part of a fixed expression.

Indeed, CALD does list this usage but adds caveats:

novel [medical ... specialized]

used to refer to a new strain (= type) of a virus that has not been seen before:

• The COVID-19 pathogen is a novel coronavirus.
• A novel virus in swine is closely related to the human hepatitis E virus. [6 Oct 2020; Proc NAS]

However, 'novel' in this sense has been around since at least the 1970s, when the terms 'novel proteins' and 'novel compounds' were often met in scientific papers. Perhaps 'new to science' is a good synonym.

In general usage, the difference between these two fairly close synonyms is brought out by this statement from Vocabulary.com:

novel If something is so new and original that it's never been seen, used or even thought of before, call it novel.

There is an emphasis on originality, freshness, a new idea being involved. A 'novelty item' is (aimed at being) a surprising, inventive, almost ingenious knickknack.

Lexico at the moment (08 Nov 2020) carries only the generally upbeat sense (doubtless the default sense prior to the 'novel virus' usage):

novel [adjective] ...

New or unusual in an interesting way.

• He hit on a novel idea to solve his financial problems.

Though 'novel virus' and 'novelty item' are terms evoking very different scenarios.

Solution 2:

A novel virus is not necessarily new. Wikipedia says

A novel virus is a virus that has not previously been recorded. It can be a virus that is isolated from its natural reservoir or isolated as the result of spread to an animal or human host where the virus had not been identified before. It can be an emergent virus, one that represents a new virus, but it can also be an extant virus that has not been previously identified.

Merriam-Webster says

novel

1 b not previously identified
transmission of a novel coronavirus, a novel genetic mutation

Solution 3:

Here is a new car

Here is a novel car

Solution 4:

Also worth mentioning that "novel" is a jargon term of patent law with a particular legal weight. Many claims of "novel" are made in scientific papers with an eye to patent law. Which has meant that "novel" has become the common term in science for a never-seen-before discovery or invention, as opposed to the use of a different adjective with a similar meaning, such as "new".

Solution 5:

In French, such a difference between "nouveau / nouvelle" (novel ?) and "neuf / neuve" (new) may exist.

As to follow the washing machine example given by @BoldBen: "the three millionth [one] of a given model to leave the factory is certainly new", hence translates as "une machine [à laver] neuve" (as opposed to second-hand) and not "une nouvelle machine". I checked the etymology of "novel", and indeed it comes from French, so it may be a clue.