This news article reports a yaagam/yajna being conducted to "to alleviate Covid-19 effect". Is this a legitimate reason i.e., sanctioned by Hindu scriptures, to conduct a yajna?
A yagam of 12 days – named Sri Lakshmi Narayana Maha yagna - would be conducted, using 1.5 lakh litres of ghee. “This yagna is to alleviate Covid-19 effect and also to free us from other viruses like hatred, inequality,” Chinna Jeeyar says.
First of all, Chinna Jeeyar, the Acharya you have quoted, is a very well known Vaishnava Acharya of his Sampradaya. Acharyas like him know much more about scriptures than the users of this site. So if he is doing the Yajna it is already understood that it has the sanction of scriptures.
Secondly, to answer -- yes it can be done for any reason, for seeking any remedy.
The purpose we are doing a Yajna, and the type of fruits we want in return, is stated in Sankalpa which is chanted before all Yajnas (or Hindu rituals).
Quoting from the book "Theory and Practice of Hindu Ritual (vol.2)":
All sacrifices have three aspects: —
uddeśa (intention or direction), viz. the god for whom the sacrifice is meant;
tyāga (what is given up); viz. the offerings or the havis; and
prakṣepa (the prescribed spot or particular fire where the offerings are delivered).
The Purpose of Yajña In Śukla Yajur Veda-Saṁhita (chapter 18;1;2 and 6) there is a detailed account of the various benefits that result from the performance of a sacrifice (yajñena kalpantām). Among such benefits are abundance of food (vāja), wealth (prasava), efficiency in work (prayati), mental acuity (dhiti), enlightenment (jyoti), physical vigour (ojas), longevity (dirghāyu), health (anāmaya), tranquility (śarma), fearlessness (abhaya), friendship (anamitram), and sound sleep (sukham śayanam ). The sacrificer prays that his life may become fulfilled by the sacrifice that he performs (ibid 9, 21 'ayur yajñena kalpatam
So, as you can see, one of the purposes of conducting a Yajna is attaining health (or freedom from diseases). Also, while chanting the Sankalpa Mantram, they will have to modify it to include the desire which here is "to alleviate Covid 19 effect".
Sankalpa, the purpose behind performing particular ritual (like Puja, Homa etc.) has to stated loudly before starting the ritual. For example, quoting from the article Kumbha Abhishekam :
The auspicious ceremony began early in the morning at 5 o’clock with Nadaswaram by temple musicians of Kancheepuram, specially invited for this occasion. Along with this, the priests started preparations for Hawanas and Pujas in the newly built Yajnashala of the temple complex. After performing an elaborate Sankalpa at the Yajnashala, all the Sannyasis/disciples of Gurudev and the priests led by Shri Kamakoti Shastriji, proceeded to Gurudev’s room in the nearby Omkarananda Shanta-Durga Mandir of the Ashram to seek His Blessings and Permission for the Mahakumbhabhishekam ceremony.
So, there is nothing wrong in the process of trying to control a pandemic with the help of performance of Yajnas. How effective will it be is a different question however. But one can desire virtually anything. It has to be mentioned clearly in the Sankalpa that's all.
