Perhaps you should try some spelling variations:
"improving vaccine performance risk." » "improving machine performance risk.", "improving masculine performance risk.", "improving baseline performance risk.", "improving teaching performance risk.", "improving accident performance risk.", "improving access performance risk.", "improving vaccine performance 2019."
"improving guiding performance review." » "improving building performance review.", "improving buildings performance review.", "improving using performance review.", "improving guiding performance evidence.", "improving living performance review.", "improving reading performance review.", "improving spending performance review."

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving vaccine performance risk.") or ("improving guiding performance review."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving vaccine performance risk.") OR ("improving guiding performance review."))*.
• Removing quotes may allow a broader search: (( improving vaccine performance risk. ) or ( improving guiding performance review. ))*.