Communication during a public health emergency event is a complex process. The objective of the present systematic review was to examine the extant literature related to such events to answer the following question: What are the best ways to communicate uncertainties to public audiences, at-risk communities, and stakeholders? The search for literature was in multiple languages; it had a focus on English and included Arabic, Chinese, French, Russian, and Spanish. The search included published studies, grey literature, and media reports. The found literature was narrowed to select only data-based primary studies, which were classified into four methodological streams: Quantitative-comparison groups; Quantitative-descriptive survey; Qualitative; and Mixed-method and case study. Study characteristics extracted from individual studies of all methodological streams included study method, country focus, disaster/ emergency type, disaster/ emergency phase, and at-risk/ vulnerable population. Study findings for all methodological streams were extracted as narrative findings. The synthesis of findings was done in two stages. In the first stage, findings from individual studies were synthesized within methodological streams and then these within-method synthesized findings were evaluated for certainty/ confidence using appropriate tools. In the second stage, the within-method synthesized findings were synthesized across methodological streams to develop an overarching synthesis of findings. In both the within-method and across-method stages, the synthesis of findings included subgroup analyses related to type of emergency event, phase of emergency event, country of emergency event, and presence of vulnerable population. The last two subgroups allowed considerations of equity in the synthesized findings. For English language literature search, almost 2900 titles and abstracts were scanned, of which about 1700 full-texts were quickly read. After this, 73 full-texts were downloaded, of which 33 data-based primary studies were selected for data extraction. For Arabic, Chinese, French, Russian, and Spanish languages all together almost 8600 titles and abstracts were scanned, of which about 900 full-texts were downloaded, of which 16 were fully read and then 13 data-based primary studies were selected for data extraction. The results showed that country coverage was of mostly high and middle-income countries in Asia, Europe, North America, and Oceania. The event most covered was infectious disease; other relatively common events included flood and earthquake. The final set of across-method synthesized findings showed that uncertainty in public health emergency events is a multi-faceted concept with multiple components. There is general agreement among experts and researchers, though with some exceptions, that communication by authorities to the public should include explicit information about uncertainties associated with events. However, it is important to ensure that the information provided is consistent and not contradictory, and is presented clearly and in an easy to understand manner. The findings also showed that uncertainty related to public health emergency events requires a distinction between uncertainty experience and uncertainty information. It is likely that uncertainty is experienced and uncertainty information is understood and misunderstood in the same general ways by both the public and experts. The experience of uncertainty may be a defining feature of a public health emergency event not only for the public and experts, but for the media as well as it has to often deal with contradictory and inconsistent uncertain information.