Berkshire disclosed $5.7 billion of new healthcare stakes, including more than $1.8 billion each in Abbvie Inc, Bristol-Myers Squibb Co, Merck & Co and $136 million in Pfizer Inc.
Shares of the drugmakers rose in after-hours trading.
The filing signals where Buffett and his portfolio managers Todd Combs and Ted Weschler see value. Buffett normally handles large investments for Berkshire's $245.3 billion stock portfolio himself.
"COVID-19 has made us think differently about healthcare," said James Armstrong, president of Henry H. Armstrong & Associates in Pittsburgh, which owns Berkshire stock.
"The sector has become more efficient as big drug companies partner with smaller, inventive rivals," Armstrong added. "But you will always need companies with scale for manufacturing and distribution, including vaccines with global application."
Berkshire did not immediately respond to a request for comment.
During the third quarter, Berkshire also took a $276 million stake in wireless phone company T-Mobile US Inc.
It sold a $1.3 billion stake in Costco Wholesale Corp, which benefited as people stocked up on groceries and home supplies during the pandemic, and pared holdings in four banks: JPMorgan Chase, Wells Fargo, PNC and M&T.
The stake in JPMorgan, where Combs is a director, fell by 96%.
Berkshire also bought more stock in the grocer Kroger, sold some stock in mining company Barrick Gold, and confirmed it bought more Bank of America stock while trimming its Apple stake.
