It has been a rough five days for Vertex Pharmaceuticals. The FDA placed a partial clinical hold on a key next-generation hepatitis-C drug at the end of last week, and this week's second-quarter earnings were a mixed bag, sending shares lower still.

In this video, health-care analyst David Williamson focuses on VX-135's clinical hold and why investors should be concerned given the drug class' checkered history, but ultimately why it doesn't damage the real reason for owning shares of Vertex.

Follow David on Twitter: @MotleyDavid.

David Williamson owns shares of Johnson & Johnson. The Motley Fool recommends Gilead Sciences, Johnson & Johnson, and Vertex Pharmaceuticals and owns shares of Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.