A journal needs to be in circulation for at least 2 years before it can be considered eligible to have an impact factor. This means it can receive an impact factor not earlier than its third year of publication. Read about how the impact factor is calculated for a journal whose title changes.
You can verify this by looking for the impact factor of Human Vaccines & Immunotherapeutics in publicly available versions of Journal Citation Reports (JCR) from previous years. If you check the impact factor listings in JCR 2012 and JCR 2013, you’ll see that there’s no impact factor mentioned for Human Vaccines & Immunotherapeutics. However, the earlier title, Human Vaccines, does appear in the 2012 and 2013 reports, with an impact factor assigned to it. This is because the impact factor of the old title continues to be calculated for 2 years after the name change, until the new title is eligible for an impact factor.
Thomson Reuters releases a new set of impact factors in its JCR every June, and those impact factors apply for the previous year. For example, the website of Human Vaccines & Immunotherapeutics shows the 2013 Impact Factor (3.643), which was released in the 2014 JCR. It is difficult to predict the precise date of JCR release but it is reasonable to expect that if your article is published after the release of the updated impact factors then the new listing will apply. Until the 2015 JCR is released, the 2014 JCR listing (IF 3.643) will be valid.
It's not clear why you found a discrepancy in the current impact factor for Human Vaccines & Immunotherapeutics, but it's best to refer to the journal website or the official Thompson Reuters JCR for accurate impact factors.
Hope this helps!