JSFiddle - React, Tailwind, and code Playground

HTML

<select name="bloodsVaccinesTypesBloods" id="bloodsVaccinesTypesBloods" class="bloodsVaccinesDropdown bloodsVaccinesTestTypes bloodsVaccinesTypesBloods">

    <optgroup label="Hepatitis B">
        <option value="12" selected="selected">Antigen blood test</option>
        <option value="13">Core antibody blood test</option>
        <option value="10">Vaccine (course of 3 vaccines & 5 year booster)</option>
        </optgroup>
    <optgroup label="Hepatitis C">
        <option value="14">Antibody blood test</option>
        <option value="15">RNA blood test</option>
    </optgroup>
</select>

JavaScript

$('#bloodsVaccinesTypesBloods').focusout(function(){
  $('#bloodsVaccinesTypesBloods > optgroup').each(function() {
	 $(this).children().each(function() {
 	 $(this).attr('label',$(this).parent().attr('label')+ ' - ' + $(this).text());
  });
  });
});
    
$('#bloodsVaccinesTypesBloods').focus(function(){
       $('#bloodsVaccinesTypesBloods > optgroup').each(function() {
					$(this).children().each(function() {
  					$(this).attr('label',$(this).text());
					 });
				});
});