Learn Hardcore Portuguese (Portugal): O médico explicou que a vacina não elimina todo o risco, mas reduz muito o contágio. - The doctor explained that the vaccine does not eliminate all risk, but greatly reduces contagion.

In Portuguese you normally need an article with a singular, countable noun used as the subject. So O médico explicou… is the natural way to say The doctor explained….

O médico = the doctor (a specific doctor understood from the situation, e.g. your doctor, the hospital doctor).
Um médico explicou… = a doctor explained… (some doctor, not specified which one).

Leaving the article out (*Médico explicou…) is wrong in normal sentences.
Note also that médico is masculine, so it takes o; vacina is feminine, so it takes a.

Why is it a vacina with the definite article if we are speaking in general about vaccines?

Portuguese very often uses the definite article to talk about things in a general way, much more than English.

Here a vacina can mean:

a specific vaccine already known from context (for example, the COVID vaccine), or
vaccines of that kind in general, as a concept.

English might say vaccination reduces risk without an article. In Portuguese, a vacina reduz o risco is the normal, idiomatic way to say this.

Could we say O médico disse que… instead of O médico explicou que…? What is the difference?

Both forms are grammatical, but they are not the same in meaning.

disse (from dizer) = said / told; it only reports that he stated something.
explicou (from explicar) = explained; it suggests he clarified how it works, maybe with reasons or details.

So O médico explicou que… implies an explanation, not just passing on information. O médico disse que… is more neutral and less focused on clarifying.

Why do we need que after explicou? Can it be omitted like English often omits that?

In Portuguese, when a verb like explicar, dizer, afirmar, lembrar is followed by a whole clause, you normally must introduce that clause with que.

O médico explicou que a vacina não elimina… = correct.
*O médico explicou a vacina não elimina… = incorrect.

Unlike English, you cannot normally drop que in this structure; leaving it out sounds ungrammatical.

Why is the negation não placed before elimina? Could we say elimina não?

Standard Portuguese places não immediately before the conjugated verb: não elimina, não reduz, não vai eliminar, and so on.

Putting não after the verb here (*elimina não todo o risco) is wrong. You might see não later in the sentence only in very colloquial emphatic patterns or set expressions, not in a straightforward declarative sentence like this one.

How does todo o risco work here? What is the difference between todo o risco and just risco or todo risco?

todo o risco = all the risk / the whole risk – the idea is complete removal of risk.
o risco = the risk, without emphasising totality.
todo risco (without the article) would mean every risk / each risk, but with this noun that version is rare and sounds odd in European Portuguese.

So não elimina todo o risco means it does not eliminate the risk completely.

Why is it o contágio? Is contágio countable, and could we say os contágios or leave the article out?

Here o contágio is being used as an uncountable noun meaning contagion / transmission in general. With this meaning, the definite article is normal: reduzir o contágio = to reduce contagion.

You can say os contágios when you really mean individual contagion events or cases, but that is rarer and more technical.
Leaving the article out (*reduz muito contágio) would sound unnatural in European Portuguese in this sentence.

How does muito work in reduz muito o contágio? Could we say reduz o contágio muito instead?

Here muito is an adverb modifying the verb reduz; it tells us how much the vaccine reduces contagion (reduces it a lot). The standard order is verb + adverb + object: reduz muito o contágio.

You might occasionally hear reduz o contágio muito, but it sounds less natural and can be a bit ambiguous. For clear, standard European Portuguese, reduz muito o contágio is the best and most idiomatic order.

Why are the verbs in the second part in the present (não elimina, reduz) if explicou is in the past?

We use explicou (past) because the act of explaining happened in the past. But we keep não elimina and reduz in the present because they express a general fact that is still true now.

Past: O médico explicou… = at some point he explained.
Present: …que a vacina não elimina…, mas reduz… = this is a current, ongoing truth about the vaccine.

If you said explicou que a vacina não eliminava todo o risco, you would be anchoring the whole statement in the past, suggesting it might only apply to that past situation.

Is mas the only possible word for but here? Could we use porém or no entanto instead?

Mas is the most common and neutral word for but and fits perfectly here.

Other options:

…não elimina todo o risco, porém reduz muito o contágio. – correct, but a bit more formal / written.
…não elimina todo o risco; no entanto, reduz muito o contágio. – sounds like however in English, usually with a pause or new sentence.

In everyday spoken European Portuguese, mas is by far the most natural choice.

Could we reverse the order of the two ideas and say …que a vacina reduz muito o contágio, mas não elimina todo o risco?

Yes, that is also correct and natural:

O médico explicou que a vacina reduz muito o contágio, mas não elimina todo o risco.

Both versions are grammatical. Changing the order only changes the emphasis: the original starts with the limitation (does not eliminate all risk), then adds the benefit; the reordered version starts with the benefit, then mentions the limitation.

Is this sentence specifically European Portuguese, or would Brazilians say it the same way?

The sentence is fully natural in both European and Brazilian Portuguese. There is nothing in the grammar or vocabulary that is restricted to Portugal.

A Brazilian speaker might say exactly the same thing, or might choose a small variation such as …mas reduz muito o risco de contágio, but the structure and word choices here work fine in both varieties.

How do you pronounce the key words in European Portuguese?

Approximate European Portuguese pronunciations (IPA), with stressed syllables in capitals in the spelling:

médico (MÉ-di-co): /ˈmɛ.ðiku/ – mé like English meh, di like a soft jee, final -co like koo.
explicou (ex-pli-COU): /ʃpliˈko/ – initial ex- as /ʃp/, stressed -cou like co in cold but shorter.
vacina (va-CI-na): /vɐˈsinɐ/ – ci like see, unstressed a and -na are very short /ɐ/.
elimina (e-li-MI-na): /ɨliˈminɐ/ – stress on mi, initial e is a reduced central vowel /ɨ/.
contágio (con-TÁ-gio): /kõˈtaʒju/ – nasal con- (similar to on in French bon), stressed tá, final -gio like zhoo.

In fast European speech, many unstressed vowels are shorter and more closed, but these are good target pronunciations.