In 2004, pneumonia killed more than 2 million children worldwide, according to the World Health Organization. By 2015, the number was less than 1 million.

Better access to antibiotics and improved nutrition account for part of the decline. But scientists say it's mostly due to vaccines introduced in the early 2000s that target up to 23 of the most deadly forms of the bacterium that causes pneumonia, Streptococcus pneumoniae.

Now, a new vaccine under development could deal another blow to the disease, lowering the number of deaths even further by targeting dozens of additional strains of S. pneumoniae, and anticipating future versions of the bacteria responsible for pneumococcal disease, which includes sepsis and meningitis.

The vaccine provoked an immune response to 72 forms of S. pneumoniae  including the 23 mentioned above  in lab tests on animals, according to new research published in the journal Science Advances. The study represents the "most comprehensive" coverage of pneumococcal disease to date, researchers say.

The limitation of existing vaccines

Each strain of S. pneumoniae contains unique polysaccharides. Vaccines such as Prevnar 13 and Synflorix connect these sugars -- by the sharing of an electron -- to a protein called CRM197. The process, known as a covalent bond, creates a potent vaccine that prompts the body to find and destroy bacteria before they colonize the body.

While effective, creating covalent bonds for each strain of S. pneumoniae is time-consuming and expensive. Plus, this type of immunization, known as a conjugate vaccine, prompts the body to eliminate each of the targeted bacteria types  regardless of whether the bacteria is idle or attacking the body.

Another vaccine, Pneumovax 23, contains sugars of 23 of the most common types of S. pneumoniae. However, the immune response it provokes is not as strong as Prevnar because the sugars are not covalently linked.

How the new vaccine works

Varieties of S. pneumoniae not covered by current immunizations are responsible for a small portion  for example, 7 to 10 percent among U.S. children of pneumonia, meningitis and other cases of pneumococcal disease.

But officials worry that will change, as these less common forms  and, potentially, yet-to-be discovered antimicrobial resistant strains  replace the 23 more common types targeted by current immunizations.

According to results from the study, the new vaccine provokes a strong immune response (comparable to Prevnar) and is engineered in a way that makes it easy to add sugars (like Pneumovax) for a broad immune response.

Key to the technology is a liposome  a tiny liquid-filled bubble made of fat  that acts as a storage tank for the sugars. Because the sugars are not covalently bonded, it's possible that the liposome could host all of the sugars that identify individual strains of S. pneumoniae.

The research team added proteins at the surface of the liposome (also non-covalently) which, together with the sugars, provoke immunotherapy. According to tests performed on mice and rabbits, the new vaccine stimulated an immune response to 72 of the more than 90 known strains of S. pneumoniae. In many cases, it outperformed Prevnar and Pneumovax.