US law allows the courts and Congress to subpoena Fauci to testify. So, he has to testify.

But: The 5th Amendment says he doesn't have to testify if he might incriminate himself in a criminal case. This trumps the law. In constitutional rochambeau: Constitution beats statute. So, he doesn't have to testify.

But, but (and this is your question, I think): The pardon makes it impossible for him to be prosecuted criminally, so the 5th Amendment, which deals with criminal cases, does not apply. So, he has to testify.

But, but, but: The 5th Amendment extends to state criminal cases, but the pardon extends only to federal prosecutions. So, he may be invoking the 5th to avoid incriminating himself in state criminal court. So, he doesn't have to testify.

But, but, but, but: If someone invokes the 5th inappropriately (e.g., they say they cannot testify without incriminating themself, but Congress thinks they can testify freely), they can be found in contempt. I'm not too familiar with how Congressional contempt proceedings are handled. I know that people have gotten away with contempt for improperly pleading the 5th before. For example, Lois Lerner was found in contempt for pleading the 5th regarding using the IRS to target conservative organizations. Nothing happened to her because the Obama DOJ dropped it. (She had even publicly apologized for targeting conservative organizations, and still the Obama DOJ said she could invoke the 5th.) If there were a contempt proceeding, I sure would like for Fauci to try to identify a state statute under which he could be prosecuted criminally, to justify his invocation of the 5th. Something tells me nothing will happen. I just don't have too much faith in our Congress (regardless of which party controls it.