You already have the keywords for each of the P, I ,C & O. However, not all studies used these keywords and you may not find the relevant paper only with these. Hence, it is important to think about other keywords with the same meaning (synonym) to increase your chance of getting relevant papers in your search results. You can try to search for the synonyms / alternative keywords for each of the PICO by:
 

• using online dictionaries eg. Merriam-webster, thesaurus or Cambridge dictionary
• searching on a search engine such as Google Scholar using the initial keywords, then, scanning the titles of articles of the search results to look for alternative words and phrases
• examining relevant abstracts or articles for alternative words or phrases.

Write down the words and phrases that you feel similar to your initial keywords and add them to your PICO.

*Tips: Use inverted commas to find the exact phrase instead of individual word, eg. "long needle"

For the purpose of the example used in this RLO, please drag a synonym to each of the categories below:  

• Infant
• Needle size
• Needle size
• Swelling
• Pain
• Mother
• Anxiety
• Vaccination
• Injection