Sprutan innehåller vaccinet.

Because spruta is the basic dictionary form, meaning syringe / injection / shot depending on context, while sprutan is the definite singular form: the syringe or the shot.

• spruta = a syringe / a shot
• sprutan = the syringe / the shot

Since the sentence talks about a specific one, Swedish uses sprutan.

For the same reason: vaccin is the basic form, while vaccinet is the definite singular form.

• vaccin = a vaccine
• vaccinet = the vaccine

So both nouns in the sentence are definite:

• sprutan = the syringe
• vaccinet = the vaccine

In Swedish, the definite article is usually attached to the end of the noun instead of being a separate word.

So English:

• the syringe
• the vaccine

becomes Swedish:

• sprutan
• vaccinet

This is one of the biggest differences from English. Swedish often says the-noun as one word.

Because Swedish nouns belong to different grammatical genders.

• spruta is an en-word
• vaccin is an ett-word

That affects how the definite form is made:

• en spruta → sprutan
• ett vaccin → vaccinet

A useful way to learn Swedish nouns is to memorize them with their article:

• en spruta
• ett vaccin

Innehåller is the present tense of the verb innehålla, which means to contain.

So:

• innehålla = to contain
• innehåller = contains / is containing

In this sentence, it simply means contains.

No, this is the normal word order for a simple statement in Swedish:

• Sprutan = subject
• innehåller = verb
• vaccinet = object

So the structure is:

subject + verb + object

That is very similar to English:

• The syringe contains the vaccine.

If you changed the order, the meaning or sentence type would change. For example:

• Innehåller sprutan vaccinet? = Does the syringe contain the vaccine?

Usually, you just have to learn it with the noun. There are some patterns, but there are many exceptions.

That is why Swedish learners are often taught to memorize nouns like this:

• en spruta
• ett vaccin

This is much more useful than memorizing only spruta or only vaccin.

A good general rule:

• most Swedish nouns are en words
• fewer are ett words

But for any specific noun, a dictionary or repeated exposure is the safest guide.

Yes. Spruta can mean different but related things depending on context, such as:

• syringe
• injection
• shot

So in real-life use, sprutan might sometimes refer to the physical syringe, and sometimes more loosely to the shot itself.

In this particular sentence, the most literal reading is the syringe contains the vaccine.

Not in normal Swedish for this meaning. That would sound wrong or very unnatural.

Here, Swedish needs the definite forms because the sentence refers to specific things:

• Sprutan innehåller vaccinet.

If you wanted a more general or indefinite meaning, you would change the sentence differently, for example:

• En spruta innehåller vaccin. = A syringe contains vaccine.
• Sprutor innehåller vaccin. = Syringes contain vaccine.

So you cannot usually just remove the endings without changing how the whole sentence works.