Meta acknowledged that Threads is intentionally blocking the search terms.
Meta acknowledged in a statement to The Washington Post that Threads is intentionally blocking the search terms and said that other terms are being blocked, but the company declined to provide a list of them. A search by The Post discovered that the words “sex,” “nude,” “gore,” “porn,” “coronavirus,” “vaccines” and “vaccination” are also among blocked words.
“The search functionality temporarily doesn’t provide results for keywords that may show potentially sensitive content,” the statement said, adding that the company will add search functionality for terms only “once we are confident in the quality of the results.”
Lucky Tran, director of science communication at Columbia University, discovered this himself when he attempted to use Threads to seek out research related to covid, something he says he does every day. “I was excited by search [on Threads],” he said. “When I typed in covid, I came up with no search results.”
Other public health workers criticized the company’s decision and said its timing was especially poor, given the current coronavirus uptick. Hospitalizations jumped nearly 16 percent in the United States last week and have been rising steadily since July, according to CDC data, though they remain less than what they were for the comparable week a year ago. Deaths are less than a quarter of what they were year to year, CDC statistics show.
(OP: Sorry, paywall, can't find another source yet. Someone got an archive?)
I'm gonna defend meta here so hold with me for a bit.
Moderating bad actors is now impossible. They can, and do, produce so much content that any level of moderation is too expensive; even for meta. The scale of idiots to non idiots is biased heavily towards idiots. A slightly different version of "a lie can travel around the world before the truth gets their pants on".
Meta, Twitter, google, will not save us from the bad actors. Even if they committed millions to the effort. Nor are they willing to risk litigation from the idiots in the GOP. So guess what? The only sustainable practice for them is to block.
The fact that this will prevent dissemination of legit info from the government and professionals is a problem society created by falling into the tolerance "paradox" trap.
I’d like to believe that Meta is acting in good faith, as you seem to be assuming, but given their history, I just am not willing to put any faith in that.  And I’m certainly not alone in feeling that way. 
What an insane decision. Deciding you won't host adult content on your platform is one thing. Saying "well you can't talk about COVID here" is something else entirely.
It's a problem of scale. Places like Threads and X are too large to do any real moderation as we think of it. Hell, Reddit only barely manages it with unpaid volunteers and that is highly uneven in its results. The sheer volume and speed of posting on those platforms would make it an absolute nightmare to try to do.