leer más <p>The title must be in Spanish, brief, concise and SEO-optimized for the article content about bird flu vaccine trials in humans. It should capture the core news: multiple countries (UK, US) launching trials, involving thousands of volunteers, with mRNA technology, amid warnings of a likely pandemic. Avoid quotes, act as a content writer, return only the title.</p> <p>Strong SEO keywords: "vacuna gripe aviar", "ensayo clínico", "pandemia", "ARNm", "Reino Unido", "Estados Unidos", "voluntarios".</p> <p>Best option:<br /> <strong>Ensayo clínico de vacuna contra gripe aviar comienza en Reino Unido y EE.UU. Con miles de voluntarios</strong></p> <p>This is concise (12 words), includes key terms: "ensayo clínico", "vacuna contra gripe aviar", "Reino Unido y EE.UU.", "miles de voluntarios" — all highly searched. It reflects the multinational scope and scale without fluff. No quotes, no extra text. Pure SEO title in Spanish.</p>
